WILLIAM  ESTY 
Head  of  Department  of  Electrical  Engineeiing,  Lehigh  University. 


ALTERNATING-CURRENT 
MACHINERY- 


A  PRACTICAL  TREATISE  ON   ALTERNATING-CURRENT 

PRINCIPLES   AND   SYSTEMS,    COMMERCIAL   TYPES 

OF  ALTERNATORS,  SYNCHRONOUS  MOTORS, 

TRANSFORMERS,  CONVERTERS,  INDUCTION 

MOTORS,  SWITCHBOARD  AND  STATION 

APPLIANCES,  ETC. 


By 

WILLIAM  ESTY,  S.B.,  M.A. 
>  i 

HEAD   OF    DEPARTMENT   OF  ELECTRICAL  ENGINEERING, 
LEHIGH   UNIVERSITY 


ILLUSTRATED 


CHICAGO 

AMERICAN  SCHOOL  OF  CORRESPONDENCE 
1912 


COPYRIGHT  1912  BY 
AMERICAN  SCHOOL  OP  CORRESPONDENCE 


Entered  at  Stationers7  Hall,  London 
All  Rights  Reserved 


CONTENTS 


ALTERNATING=CURRENT  PRINCIPLES 

ALTERNATING  ELECTROMOTIVE  FORCES  AND  CURRENTS 

Page 

Simple  alternator 1 

Variations  of  electromotive  force 3 

Relations  between  speed  and  frequency 3 

Advantages  and  disadvantages  of  alternating  currents 4 

Comparison  of  direct-  and  alternating-current  problems 7 

Physical  basis  for  the  differences  between  D.  C.  and  A.  C.  calcula- 
tions    10 

Graphical  representation  of  alternating  electromotive  forces  and  cur- 
rents    12 

Average  and  effective  values  of  E.  M.  F 14 

Instantaneous  and  average  power 15 

Harmonic  electromotive  forces  and  currents 16 

Relation  between  maximum  and  effective  values 24 

Inductance 27 

Capacity 30 

Capacity  of  condensers 31 

Fundamental  equations  of  the  A.  C.  circuit 32 

Electrical  resonance 41 

Miscellaneous  considerations . .  47 


MEASURING  INSTRUMENTS 

INDICATING   INSTRUMENTS 

Hot  wire  ammeter  and  voltmeter 60 

Electrostatic  voltmeter, 64 

Electrodynamometers 72 

Induction  instruments 75 

Wattmeter 77 

INTEGRATING   INSTRUMENTS 

Thomson  Watt-hour  meter 82 

Induction  Watt-hour  meter 85 

Directions  for  reading  Watt-hour  meter  dials 93 

Calculations  of  customers'  bills 95 

Spark  gauge 95 


257953 


2  CONTENTS 

ALTERNATORS 

Page 

Fundamental  equation  of  alternator 97 

Armature  reaction 104 

Armature  inductance 106 

Electromotive  force  los"  in  armature  drop 107 

Alternator  regulation 120 

Field  excitation 109 

POLYPHASE   ALTERNATORS   AND   SYSTEMS 

Limitations  of  single-phase  system 118 

Two-phase  alternator 118 

Three-phase  alternator 121 

Y-connected  armatures 124 

A-connected  armatures 125 

Receiving  circuits  to  three-phase  mains 126 

Summary  of  electromotive-force  and  current  relations  for  A  and  Y 

connections 127 

MEASUREMENT   OF   POWER 

Balanced  systems 129 

Unbalanced  systems 131 

ARMATURE   WINDINGS 

Classification 135 

Single-phase  windings. 142 

Two-phase  windings 143 

Three-phase  windings 143 

Commercial  Types 

REVOLVING-ARMATURE  ALTERNATORS 

Fort   Wayne    single-phase 147 

Westinghouse   uni-coil    armature 150 

Westinghouse  armature   with  distributed  winding 151 

Field   structure    for    Westinghouse   180-kw.    alternator 152 

General  Electric  three-phase  alternator 152 

REVOLVING-FIELD   ALTERNATORS 

Construction 154 

Water-wheel-driven  alternators 165 

Steam-turbine-driven  alternators 169 

Economy    Factors 

CONDITIONS   AFFECTING  COST 

Speed 179 

Voltage •••  179 

Regulation 

Frequency • 180 


CONTENTS  3 

POWER  LOSSES 

EFFICIENCY  Page 

Practical  and  ultimate  limits  of  output 182 

Influence  of  power  factor  upon  output 183 

RATING  AND   OVERLOAD   CAPACITIES 

American  Institute  rules 184 

TESTING 

Alternating-current  testing  in   general 188 

Insulation  testing * 188 

Determination  of  resistance  of  armature 198 

Regulation 200 

Heat  test 205 

Core  loss  and  friction  test 209 

Calculation  of  efficiency .  214 

SYNCHRONOUS  MOTORS 

Any  alternator  a  synchronous  motor 216 

Advantages 218 

Disadvantages : 219 

Comparison  of  synchronous  and  D.  C.  motors 219 

Starting  the  motor 220 

Hunting  action 223 

Torque  and  power  output 225 

Field  excitation  and  power  factor 226 

Use  as  a  condenser 227 

TESTING 

Phase  characteristic 229 

Pulsation  test 230 

Break-down  test 231 

Self-starting  test 231 

TRANSFORMER 

Description 234 

Physical  action 234 

Automatic  action  of  the  transformer 236 

Ideal  and  practical  transformer 236 

Maximum  core  flux 237 

Ideal  transformer  action  graphically  represented 239 

Influence  of  coil  resistance  and  magnetic  leakage 240 

Performance 241 

CONNECTIONS 

Parallel-constant-voltage  transformers 242 

Autotransformer. .                                                                                ....  251 


4  CONTENTS 

IN   POLYPHASE  SYSTEMS  Page 

Two-phase  system 255 

Three-phase  system 255 

Transformers  with  compound  magnetic  circuits 257 

Phase  transformation 257 

PRACTICAL  CONSIDERATIONS 

Transformer  losses 262 

Transformer  efficiency 263 

Transformer  regulation 266 

Rating  of  transformers 267 

COMMERCIAL   TYPES 

Core  type 271 

Shell  type 283 

Three-phase    transformers 289 

Cooling  of  transformers '.  .  .  292 

Series  or  current  transformers 297 

Constant-current  transformers 299 

Transformer  fuse  blocks 305 

Mounting  of  outdoor  transformers 308 

TESTS 

Heat  test 308 

Core-loss  and  exciting-current  test 310 

Resistance  of  coils 310 

Impedance •  •  •  •  311 

Regulation 313 

Efficiency  calculation 314 

Polarity  test 315 

CONVERSION  OF  ALTERNATING   INTO  DIRECT  CURRENT 

Rectifying  commutator 317 

Aluminum  valve  rectifier 318 

Mercury-vapor  arc  rectifier 320 

Rotary  or  Synchronous   Converter 

Comparison  with  direct-current  dynamo 326 

To  make  a  direct-current  dynamo  into  a  rotary  converter 327 

Three-ring    converter 329 

Four-ring  converter 329 

Six-ring    converter 330 

Multipolar  rotary  converters 330 

E.  M.  F.  relations  for  rotary  converter 330 

Current  relations  for  rotary  converter 336 


CONTENTS  5 

ROTARY  CONVERTERS   IN   PRACTICE  Page 

Uses  of  the  rotary  converters 333 

Starting  rotary  converters 339 

Oscillators  for  rotary  converters 342 

Characteristic  types  of  rotary  converters 343 

Hunting  of  rotary  converter 345 

Inverted  rotaries 347 

Control  of  direct-current  voltage 349 

Field  excitation -7 .  .  353 

Rotary  converter  with  Edison  three-wire  system 355 

Six-phase  converter 357 

Transformer  connections  for  rotary  converters 357 

TESTING   ROTARY   CONVERTERS 

Standard  tests 360 

Heat   run 361 

Motor  Generators 

Comparison  with  the  rotary  converter 365 

Use  of  motor  generators 366 

Types 367 

INDUCTION     MOTOR 

Constructive  elements 371 

Stator  windings  and  their  action 372 

Action  of 375 

Torque  and  speed 375 

Starting  resistance  in  the  rotor  windings 376 

Efficiency  and  speed 376 

Ratio  of  mechanical  to  electrical  energy  in  rotor 377 

Ratio  of  rotor  voltages  to  stator  voltages 378 

Efficiency  and  rotor  resistance 379 

Structural  details  of  a  typical  induction  motor 380 

Types  of  rotors  for  constant  and  variable  speed 381 

Behavior  at  starting  and  in  operation 385 

Typical  induction  motor  installations 389 

Single-phase  induction  motor 393 

Induction  generator 397 

Frequency  changer 397 

Comparison  of  Synchronous  Motor  and  Induction  Motor 

INDUCTION   MOTOR  TESTS 

Breakdown  test :  . 408 

Starting  torque  test 409 

Core  loss  test . .  410 


6  CONTENTS 


Impedance  test > 412 

Efficiency  test. 413 

Slip  test 414 

Performance  curves  of  an  induction  motor 416 

SWITCHBOARD  AND  STATION  APPLIANCES 

SWITCHBOARDS 

Typical  single-phase  switchboard 422 

Polyphase  switchboard 425 

Feeder  panels 430 

High  voltage  panels 433 

SPECIAL  SWITCHBOARD  APPARATUS 

Lincoln  synchronizer 433 

CIRCUIT-INTERRUPTING   DEVICES 

Fuses 435 

Air-break  switches 436 

Circuit  breakers 438 

Oil-break  switches 440 

Feeder  or  voltage  regulators 444 

Voltmeter  compensator 450 

LIGHTNING   ARRESTERS 

Effects  of  lightning 452 

Multi-gap  non-arcing  arrester .  454 

Multi-path  arrester 455 

Electrolytic-cell  lightning  arrester 456 

Combination  of  a  condenser  with  a  choke  coil 459 

APPENDIX 

PARALLEL  OPERATING  OF  ALTERNATORS 

Necessary  conditions 462 

Determination  of  relative  frequency  and  phase  coincidence 462 

Directions  for  connecting  one  alternator  in  parallel  with  another  alter- 
nator             466 

Directions  for  cutting  out  an  alternator  which  is  running  in  parallel 

with  one  or  more  alternators 466 

Index  ,                     499 


INTRODUCTION 


THIS  treatise  has  been  prepared  with  the  special  object  of  giv- 
ing the  beginner  and  the  so-called  practical  electrician  a  work- 
ing knowledge  of  alternating-current  apparatus  in  order  that 
he  may  know  how  to  install  and  operate  it  intelligently.  In  most 
cases  no  argument  is  needed  to  convince  the  practical  station  man 
of  the  advantage  of  getting  this  working  knowledge.  But  unfortun- 
ately there  seems  to  be  a  general  impression  that  no  one  who  is  not 
an  expert  mathematician  or  a  college  graduate  can  get  much  use- 
ful information  out  of  a  book  on  alternating  currents.  Knowing 
only  the  rudiments  of  algebra,  geometry,  and  trigonometry,  the 
beginner  soon  gets  beyond  his  depth  in  a  sea  of  advanced  mathe- 
matics, becomes  discouraged,  and  finally  gives  up  in  disgust.  It  is 
precisely  for  him  that  this  book  is  written.  It  contains  no  mathe- 
matics beyond  the  simplest  trigonometry;  and  great  pains  have  been 
taken  to  make  the  descriptions,  explanations,  and  proofs  simple 
and  clear.  The  reader  is  assumed  to  have  some  acquaintance  with 
the  elementary  laws  of  electricity  and  magnetism. 
C|  Graphical,  or  geometric,  methods  rather  than  analytical,  or  alge- 
braic, methods  have  been  adopted  wherever  possible,  as  will  be 
seen  in  the  repeated  application  of  the  "clock"  diagram  in  deter- 
mining the  relations  between  electromotive  forces  and  currents  in 
the  various  types  of  apparatus  discussed.  Abstract  and  arbitrary 
statements  have  been  reduced  to  a  minimum;  and  on  the  other  hand, 
a  large  variety  of  examples  and  numerical  illustrations  have  been 
used  throughout  the  text  to  reinforce  the  principles  by  concrete 
applications. 

ljln  the  opening  articles  the  reader  is  made  acquainted  with  the 
essential  features  of  the  source  of  alternating  currents — the  alter- 
nator— and,  through  it,  is  introduced  to  the  new  terms,  definitions, 
and  conceptions  which  form  the  alphabet  of  the  alternating-cur- 


rent  language.  Attention  is  early  called  to  the  characteristic  fea- 
tures of  alternating-current  problems  which  cause  them  to  differ 
from  direct-current  problems.  Indeed,  the  first  fifty  pages  may  be 
considered  the  foundation  upon  which  the  rest  of  the  book  is  built. 
After  the  fundamental  principles  of  alternating-current  working 
have  been  explained  and  illustrated,  a  description  and  discussion 
of  the  instruments  used  to  measure  alternating  current,  electro- 
motive force,  and  power,  follow  in  natural  sequence.  The  general 
plan  which  has  been  adopted  in  the  treatment  of  the  subsequent 
sections  dealing  successively  with  alternators,  synchronous  motors, 
transformers,  rotary  converters,  and  induction  motors,  is  as  follows: 
(a)  The  physical  theory  of  each  class  of  apparatus  is  explained;  (6) 
the  applications,  behavior,  and  operation  are  discussed;  (c)  the 
structural  details  of  commercial  types  are  illustrated;  (d)  tl\e  meth- 
ods used  in  making  the  usual  tests,  and  the  calculations  based  thereon 
are  explained. 

t][  In  conclusion  the  author  expresses  the  hope  that  the  reader  who 
has  thoughtfully  followed  the  elements  of  the  subject  as  here  pre- 
sented, may  be  encouraged  to  pursue  further  the  study  of  alternating- 
current  machinery  as  discussed  in  the  more  advanced  text-books. 

WILLIAM  ESTY. 


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ALTERNATING=CURRENT 
MACHINERY 


PART  I 

ALTERNATING  ELECTROMOTIVE  FORCES  AND  CURRENTS 

Simple  Alternator.  The  alternator  is  an  arrangement  by  means 
of  which  mechanical  energy  is  used  to  cause  the  magnetic  flux  from 
a  magnet  to  pass  through  the  opening  of  a  coil  of  wire  first  in  one 
and  then  in  the  opposite  direction.  This  varying  magnetic  flux 
induces  in  the  coil,  first  in  one  direction  and  then  in  the  other,  what 
is  called  an  alternating  electromotive  force,  which  in  turn  produces 
an  alternating  current  in  the  coil,  and  in  the  circuit  which  is  con- 
nected to  the  terminals  of  the  coil. 

In  the  common  type  of  alternator,  the  above-mentioned  magnet 
and  coil  move  relatively  to  each  other.  Fig.  1  shows  the  essential 
features  of  such  an  alternator.  The  poles  N,  S,  N,  S,  etc.,  of  a 
multipolar  magnet  called  the  field  magnet,  project  radially  inwards 
toward  the  passing  teeth  a  a  a  of  a  rotating  mass  A  of  laminated 
iron;  and  upon  these  teeth  are  wound  coils  of  wire  c  c,  in  which 
the  alternating  electromotive  force  is  induced.  The  rotating  mass 
of  iron  with  its  windings  of  wire  is  called  the  armature.  At  one  end 
of  the  armature  (not  shown  in  the  figure)  are  mounted  two  insulated 
metal  rings  r  r,  called  collecting  rings.  These  metal  rings  are  con- 
nected to  the  ends  of  the  armature  winding,  and  metal  brushes  b  b 
rub  on  these  rings,  thus  keeping  the  ends  of  the  armature  winding 
in  continuous  contact  with  the  terminals  of  the  external  circuit  to 
which  the  alternator  supplies  alternating  current.  No  external 
circuit  is  shown  in  the  figure. 

The  electromotive  forces  induced  in  adjacent  armature  coils 
are  in  opposite  directions  at  each  instant,  and  the  coils  are  so  con- 
nected together  that  these  electromotive  forces  do  not  oppose  each 

Copyright,  1912,  by  American  School  of  Correspondence. 


2'*'       *  A-LT£RNATING-CURRENT  MACHINERY 

other.  This  is  done  by  reversing  the  connections  of  every  alternate 
coil,  as  indicated  by  the  dotted  lines  connecting  the  coils  in  Fig.  1. 
The  electromagnetic  action  of  this  type  of  alternator  depends  only 
upon  the  relative  motion  of  field  magnet  and  armature,  and  large 
machines  are  usually  built  with  stationary  armature  and  revolving 
field  magnet.  In  the  type  of  machine  illustrated  in  Fig.  1,  the  arma- 
ture revolves  while  the  field  magnet  is  stationary.  This  type,  called 
the  revolving-armature  type,  is  generally  adopted  in  small  alternators. 
The  field  magnet  of  an  alternator  is  usually  an  electromagnet 
which  is  excited  by  a  continuous  electric  current  supplied  by  an 
independent  generator,  generally  by  an  auxiliary  continuous-current 
dynamo,  called  the  exciter.  The  exciting  current  flows  through  coils 


Fig.   1.     Diagram  of  Armature  and  Field 
of  Simple  Alternator 

of  wire  wound  on  the  projecting  poles  N,  S,  N,  S  of  the  field  magnet. 
These  coils  are  not  shown  in  Fig.  1. 

The  type  of  armature  core  shown  in  Fig.  1  is  called  the  toothed 
armature  core;  and  the  armature  winding  is  said  to  be  concentrated, 
that  is,  the  armature  conductors  are  grouped  in  a  few  heavy  bunches. 
Armature  cores  are  also  made  with  many  small  slots,  in  which  the 
armature  conductors  are  grouped  in  small  bunches.  This  type  of 
core  is  called  a  multi-toothed  core,  and  the  winding  is  said  to  be  dis- 
tributed. 

In  some  of  the  earlier  types  of  alternators  the  armature  core 
consisted  of  a  smooth,  cylindrical  mass  of  laminated  iron,  upon 


ALTERNATING-CURRENT  MACHINERY  3 

the  face  of  which  the  conductors  were  arranged  in  bands  side  by 
side,  one  layer  or  more  in  depth.  This  type  of  armature  is  called 
the  smooth-core  armature;  it  has  been  superseded  by  the  toothed 
core  type. 

Variations  of  Electromotive  Force.  Cycle.  The  electromotive 
force  of  an  alternator  passes  through  a  set  of  positive  values  while  a 
given  coil  of  the  armature  is  passing  from  a  south  to  a  north  pole 
of  the  field  magnet,  and  through  a  similar  set  of  negative  values  while 
the  coil  is  passing  from  a  north  to  a  south  pole.  The  complete  set 
of  values,  including  positive  and  negative  is  called  a  cycle. 

Frequency.  Frequency  is  equal  to  the  number  of  cycles  per 
second;  it  is  sometimes  expressed  by  stating  the  number  of  alterna- 
tions or  reversals  per  minute.  For  example,  an  alternator  having  a 
frequency  of  133  cycles  per  second  has  266  reversals  or  alternations 
per  second,  or  15,960  alternations  per  minute.  Frequencies  are 
sometimes  specified  in  alternations  per  minute,  but  specification  in 
cycles  per  second  is  the  more  usual  practice  and  is  preferable. 

alternations  per  min. 
cycles  per  sec.  =  2XQO 

Period.  The  fractional  part  of  a  second  occupied  by  one  cycle 
is  called  the  periodic  time,  or  period,  of  the  alternating  electromotive 
force  or  current. 

Let  /  be  the  frequency  in  cycles  per  second,  and  T  the  period 
expressed  as  a  fraction  of  a  second.  Then 

f=jr  0)' 

Therefore,  if  an  alternating  current  has  a  frequency  of  60  cycles 
per  second,  the  period  T  of  one  cycle  is  one-sixtieth  of  a  second. 

Relations  Between  Speed  and  Frequency.  Let  p  be  the  num- 
ber of  poles  of  the  field  magnet  of  an  alternating-current  machine; 
let  n  be  the  speed  of  its  armature  in  revolutions  per  minute;  and 
let  /  be  the  frequency  of  its  electromotive  force  in  cycles  per  second. 
Then 


60 


4  ALTERNATING-CURRENT  MACHINERY 

Examples.  1.  A  certain  alternator  has  10  poles,  and  runs  at  1,500 
revolutions  per  minute.  What  is  its  frequency? 

SOLUTION.  Substituting  10  for  p,  and  1,500  for  n,  in  equation  (2),  we 
have 

10       1500 
/=  —  X  —  —  =  125  cycles  per  second 

2.     An  alternator  is  to  run  at  600  revolutions  per  minute  and  is  to  give 
a  frequency  of  60  cycles  per  second.     What  number  of  poles  is  required? 
SOLUTION.     Solving  equation  (2)  for  p,  we  have 

_2X60X/ 

from  which,  substituting  /  =  60,  and  n  —  600,  we  have 

2X60X60 


Advantages  and  Disadvantages  of  Alternating  Currents.    The 

electric  transmission  of  a  given  amount  of  power  may  be  accom- 
plished by  a  large  current  at  low  electromotive  force,  or  by  a  small 
current  at  high  electromotive  force.  In  the  first  case  very  large 
and  expensive  transmission  wires  must  be  used,  or  the  loss  of  power 
in  the  transmission  line  will  be  excessive.  In  the  second  case  com- 
paratively small  and  inexpensive  transmission  wires  may  be  used. 
Thus  it  is  a  practical  necessity  to  employ  high  electromotive  forces 
in  long-distance  transmission  of  power. 

Example.  It  is  desired  to  transmit  1,000  kilowatts  of  power  over  a 
distance  of  10  miles,  supposing  that  a  loss  in  the  line  of  10  per  cent  of  the 
power  delivered  is  considered  permissible.  This  corresponds  to  a  loss  of  100 
kilowatts. 

SOLUTION.  —  Case  1.  Suppose  that  the  electromotive  force  at  the  re- 
ceiving end  of  the  line  is  to  be  100  volts.  Then  the  current  would  be  1,000,000 
watts  divided  by  100  volts,  or  10,000  amperes.  The  resistance  of  the  line 
must  be  such  that  the  watts  lost  in  the  line  —  namely  100,000  watts  —  would 

W 
be  equal  to  I2R,  so  that  the  resistance  R  of  the  line  must  be  —  5-,  or  0.001  ohm. 

This  would  require  two  transmission  wires  each  10  miles  long  and  33  inches 
in  diameter,  or  a  total  weight  of  175,000  tons  of  copper,  which  would  cost 
about  $52,500,000. 

Case  2.  Suppose  that  the  electromotive  force  at  the  receiving  end  of 
the  line  is  to  be  1,000  volts.  Then  the  current  would  be  1,000,000  watts 
divided  by  1,000  volts,  or  1,000  amperes.  The  resistance  of  the  line  must  be 
such  that  the  watts  lost  in  the  line  —  namely,  100,000  watts  —  would  be  equal 
to  PRy  so  that  the  resistance  R  of  the  line  must  be  0.1  ohm.  This  would 
require  two  transmission  wires,  each  10  miles  long  and  3.3  inches  in  diameter, 
or  a  total  weight  of  1,750  tons  of  copper,  which  would  cost  about  $525,000. 


ALTERNATING-CURRENT  MACHINERY 


TABLE  I 
Size  and  Cost  of  Copper  Wire — Two=Wire  System 

To  transmit    1,000   kilowatts  a  distance   of    lO^miles  (one  way)  with  a  line  loss  equal  to  10 
per  cent  of  the  power  delivered,  for_three  different  values  of  electromotive  force 


Volts  at 

Amperes 

Ohms 

Diameter 

Weight 

"Cost  of 

Receiving  End 
of  Line 

in 
Line 

in 
Line 

of 
Wire, 

of 
Wire, 

Line  Copper, 
in 

E 

7 

R 

in  Inches 

in  Tons 

Dollars 

100 

10,000 

0.001 

33 

175,000 

52,500,000 

1,000 

1,000 

0.1 

3.3 

1,750 

525,000 

10,000 

100 

10.0 

0.33 

15.5 

5,250 

Case  3.  Suppose  that  the  electromotive  force  at  the  receiving  end  of 
the  line  is  to  be  10,000  volts.  Then  the  current  would  be  1,000,000  watts 
divided  by  10,000  volts,  or  100  amperes.  The  resistance  of  the  line  must 
be  such  that  the  watts  lost  in  the  line — namely  100,000  watts — would  be 
equal  to  I2R,  so  that  the  resistance  R  of  the  line  must  be  10  ohms.  This 
would  require  two  transmission  wires,  each  10  miles  long  and  0.33  inch  in 
diameter,  or  a  total  weight  of  17.5  tons  of  copper,  which  would  cost  about 
$5,250. 

These  results  are  summarized  in  Table  I. 

Transformation  of  High  E.  M.  F.'s.  High  electromotive  forces 
are  dangerous  under  the  conditions  that  ordinarily  obtain  among 
users  of  electric  light  and  power;  and  many  types  of  apparatus, 
such  as  incandescent  lamps,  operate  satisfactorily  only  with  medium 
or  low  electromotive  forces.  Therefore,  means  must  be  provided, 
at  a  receiving  station,  for  transforming  the  power  delivered,  from 
high  electromotive  force  and  small  current  to  low  electromotive 
force  and  large  current,  if  long-distance  transmission  is  to  be  success- 
ful. This  is  called  step-down  transformation.  The  advantage  of 
the  alternating  current  over  the  direct  current  lies  almost  wholly  in 
the  cheapness  of  construction  and  of  operation,  and  in  the  high 
efficiency  of  the  alternating-current  apparatus  as  compared  with 
the  direct-current  apparatus  that  is  required  for  transformation. 

In  step-down  transformation  of  direct  current,  a  motor  takes 
a  small  current  from  the  high-electromotive-force  transmission 
mains,  and  drives  a  dynamo  which  delivers  large  current  to  service 
mains  at  low  electromotive  force.  This  apparatus,  or  its  equivalent, 
the  dynamotor,  is  expensive  to  construct;  it  requires  attention  in 
operation;  and  its  efficiency  is  never,  perhaps,  above  90  per  cent. 


6  ALTERNATING-CURRENT  MACHINERY 

The  step-down  transformation  of  alternating  currents  is  accom- 
plished by  means  of  the  alternating-cur  tent  transformer,  which  is 
described  later  on.  The  alternating-current  transformer  is  very 
much  cheaper  than  a  dynamo  and  motor  of  the  same  output;  it 
requires  no  attention  in  operation;  and  its  efficiency  under  full  load 
is  usually  greater  than  97  per  cent,  especially  in  large  sizes. 

Simple  Construction  of  A.  C.  Machines.  The  alternating  cur- 
rent has  some  minor  advantages  over  the  direct  current  on  account 
of  the  fact  that  alternating-current  machines  are  frequently  simpler 
in  construction  than  direct-current  machines.  In  particular,  the 
commutator  is  not  an  essential  part  of  an  alternating-current  gen- 
erator. Again,  in  the  case  of  the  inductor  alternator  and  the  induc- 
tion motor,  the  rotating  part  may  not  have  any  sliding  electrical 
contacts  whatever. 

Miscellaneous  A.  C.  Machines.  The  simple  single-phase  alter- 
nating current  is  not  well  adapted  to  general  power  service.  The 
single-phase  alternating-current  induction  motor  does  not  start 
satisfactorily  under  load,  in  the  case  of  large  machines,  although 
self-starting  single-phase  motors  up  to  perhaps  20  horse-power 
are  in  commercial  use,  where  neither  direct-current  nor  polyphase 
alternating-current  machines  are  available. 

The  single-phase  series  commutator  motor  within  a  few  years 
has  been  developed  especially  for  electric  railway  service  both  for 
trolley  cars  and  for  electric  locomotives.  Its  operating  characteristics, 
resembling  closely  those  of  the  direct-current  series  motor,  however, 
are  not  suitable  for  general  power  requirements.  This  type  of  motor 
is  used  on  the  electric  locomotives  of  the  New  York,  New  Haven 
and  Hartford  Railroad. 

For  uninterrupted  service  the  synchronous  motor  is  frequently 
used,  the  starting  being  effected  by  an  auxiliary  engine  or  other 
independent  mover.  The  synchronous  motor  is  not  satisfactory 
when  frequent  starting  is  necessary,  for  such  service  the  induction 
motor  being  used.  The  simple  induction  motor,  to  start  satisfac- 
torily, must  be  supplied  with  two  or  more  distinct  alternating 
currents  transmitted  to  .the  motor  over  separate  lines.  This  is 
called  the  polyphase  system  of  transmission,  and  is  reserved  for  full 
treatment  in  later  pages. 

For   some  purposes,   especially   for  the   electrolytic   processes 


ALTERNATING-CURRENT  MACHINERY  7 

used  on  a  large  scale  in  electro-chemical  works,  only  direct  current 
can  be  used.  When  power  transmitted  by  alternating  current  is 
to  be  delivered  in  the  form  of  direct  current,  the  conversion  is  effected 
by  means  of  the  rotary  converter,  motor  generator,  or  mercury  rectifier. 

Comparison  of  Direct=  and  Alternating=Current  Problems. 
Direct  Current.  In  direct-current  work  the  electrical  engineer  is 
concerned  with  the  relations  between  electromotive  force,  resistance, 
current,  and  power.  These  relations  are  determined  by  the  applica- 
tions of  the  following  laws: 

POWER  LAW  for  direct-current  circuits 

P=EI  (3) 

in  which  P  is  the  total  power  in  watts  delivered  to  a  circuit  by  a 
generator  of  which  the  terminal    electromotive    force  is  E  volts, 
when  it  produces  a  current  of  I  amperes. 
OHM'S  LAW  for  direct-current  circuits 


in  which  I  amperes  is  the  steady  current  produced  by  E  volts  acting 
on  a  circuit  of  R  ohms  resistance. 

JOULE'S  LAW  for  direct-current  circuits 

P=PR  (5) 

in  which  P  is  the  power  in  watts  expended  in  heating  a  circuit  of 
R  ohms  resistance,  when  a  current  of  /  amperes  is  forced  through 
the  circuit. 

KIRCHOFF'S  LAWS  for  direct-current  circuits. 

1.  When  a  circuit  branches,  the  current  in  the  main  circuit 
is  equal  to  the  sum  of  the  currents  in  the  separate  branches. 

2.  (a)  When  two  or  more  sources  of  electromotive  force  are 
connected  in  series,  the  total  electromotive  force  is  the  sum  of  the 
individual  electromotive  forces. 

3.  (b)  When  an  electromotive  force   acts   on   a  number  of 
elements  or  things  in  series,  it  is  subdivided  into  parts,  each  of 
which  acts  upon  one  of  the  elements,  and  the  sum  of  these  parts  is 
equal  to  the  total  electromotive  force.     For  example,  an  arc-light 
dynamo  of  which  the  terminal  electromotive  force  is  3,000  volts, 


8  ALTERNATING-CURRENT  MACHINERY 

acts  on  60  similar  arc  lamps  connected  in  series.  Neglecting  the 
resistance  of  the  connecting  wires,  each  lamp  is  acted  upon  by  one- 
sixtieth  of  the  total  electromotive  force,  or  by  50  volts. 

Alternating  Current.  In  alternating-current  work  the  electrical 
engineer  is  likewise  concerned  with  the  relations  between  electro- 
motive force,  resistance,  current,  and  power.  These  relations  are 
determined  by  the  application  of  the  same  fundamental  laws  as  in 
the  case  of  direct  currents,  but  in  more  or  less  modified  forms.* 
A  summary  of  the  fundamental  laws  of  alternating  currents  is  here 
given  simply  for  purposes  of  comparison. 

POWER  LAW  for  alternating-current  circuits 

P=  El  cos  0  (6) 

in  which  P  is  the  power  in  watts  delivered  to  a  circuit  by  an  alter- 
nator of  which  the  "effective"  terminal  electromotive  force  is  E 
volts,  when  it  produces  an  "effective"  current  of  I  amperes  in  a 
circuit,  and  cos  6  is  what  is  called  the  "power  factor"  of  the  circuit. 
OHM'S  LAW  for  alternating-current  circuits 

E 
= 


in  which  7  is  the  "effective"  current  in  amperes  produced  by  an 
"effective"  electromotive  force  of  E  volts  acting  on  a  circuit  of 
which  the  resistance  is  R  ohms,  and  the  "reactance"  is  X  ohms. 
The  expression  i/R2XX2  in  equation  (7)  is  called  the  impedance, 
and  it  is  expressed  in  ohms. 

JOULE'S  LAW  for  alternating-current  circuits 

P=PR  (5) 

in  which  P  is  the  power  in  watts  expended  in  heating  a  circuit  of  R 
ohms  resistance  when  an  "effective"  alternating  current  of  /  amperes 
is  forced  through  the  circuit. 

KIRCHOFF'S  LAWS  for  alternating-current  circuits 
1.     When  an  alternating-current  circuit  branches,  the  "effective" 
current  in  the  main  circuit  is  the  "geometric"!  (or  "vector")  sum 
of  the  "effective"  currents  in  the  separate  branches. 

*The  student  is  not  expected  to  understand  fully  the  reasons  for  the  statements  here 
given,  until  he  has  completed  Parts  I  and  II. 

•{•Alternating  electromotive  forces  and  alternating  currents  are  added  in  the  same 
way  that  forces  are  added,  that  is,  by  means  of  the  principle  known  as  the  "parallelogram 
of  forces." 


ALTERNATING-CURRENT  MACHINERY 


9 


Example.  An  alternator  A,  Fig.  2,  supplies  an  effective  current  of  / 
amperes  in  the  main  circuit,  which  divides  into  two  branches.  The'  effective 
currents  in  the  two  branches  are  7:  and  72  amperes,  respectively.  The -rela- 
tion between  7,  7^  and  72  is  shown  in  Fig.  3.  The  angles  6l  and  02  depend 


Fig.  2.     Diagram  of  a  Branched 
Alternating  Circuit 


Fig.  3.     Vector  Diagram  of 
Branched  Circuit 


upon  the  relative  values  of  the  resistance  and  reactance  of  the  respective 
branches,  as  is  explained  later.  It  is  to  be  particularly  noticed  that  the 
arithmetical  sum  of  7X  and  72  is  in  general  greater  than  7. 

2.  (a)  When  two  or  more  alternators  (or  transformer  second- 
aries) are  connected  in  series,  the  total  effective  electromotive  force 
is  the  "geometric"  (or  "vector")  sum  of  the  effective  electromotive 
forces  of  the  individual  alternators. 


MAfHS 


MA//YS 
Fig.  4.     Two  Alternators  in  Series  . 


Fig.  5.  Vector  Diagram  of  E.  M. 
for  Two  Alternators  in  Series 


Example.  Two  alternators  Av  and  At,  Fig.  4,  of  which  the  effective 
electromotive  forces  are  Ev  and  E2,  respectively,  are  connected  in  series  to 
supply  mains.  Then  the  effective  electromotive  force  E,  between  mains,  is 
the  geometric  sum  of  El  and  E2,  as  shown  in  Fig.  5.  The  angle  6  depends 
upon  the  positions,  relatively  to  the  field  magnets,  of  the  armature  coils  on 
the  respective  machines. 


10 


ALTERNATING-CURRENT  MACHINERY 


(b)  When  an  alternating  electromotive  force  E  acts  upon  a 
number  of  elements  or  things  in  series,  it  is  subdivided  into  parts, 
each  of  which  acts  upon  one  of  the  elements,  and  the  "geometric" 
(or  "vector")  sum  of  these  parts  is  equal  to  E. 

Example.  Two  coils  b  and  c,  Fig.  6,  are  connected  in  series  between 
mains  supplied  from  an  alternator,  of  which  the  effective  electromotive  force 
is  E.  Then  the  total  effective  electromotive  force  E  is  subdivided  into  two 
parts  Ei  and  Ea,  which  act  upon  the  respective  coils,  as  indicated  in  Fig.  6; 
and  the  "geometric,"  or 
in  Fig.  7.  The  angles  61 
actance  of  the  respective  coils.  It  is  to  be  particularly  noticed  that  the  arith- 
metical sum  of  Ei  and  Ez  is  in  general  greater  than  E. 

Physical  Basis  for  the  Differences  between  D.  C.  and  A.  C. 
Calculations.  The  above  mentioned  differences  between  direct- 
current  and  alternating-current  calculations  are  due  to  the  fact 
that  an  alternating  current  changes  rapidly  in  value  from  instant  to 
instant,  while  a  direct  current  is  steady  and  does  not  change  its 


1  Vector"  sum  of  Ei  and  Ez  is  equal  to  E,  as  shown 
and  62  depend  upon  the  relative  resistance  and  re- 


Fig.  6. 


Alternating   Current   Through 
Two   Coils   in   Series 


Fig.  7.     Vector  Diagram  of  E.  M.  F.  for 
Conditions  Shown  in  Fig.  6. 


direction  of  flow.     A  clear  idea  of  the  efforts  of  the  rapid  changes  of 
an  alternating  current  may  be  obtained  as  follows : 

Fig.  8  represents  an  alternator  producing  alternating  current 
in  a  circuit  of  wire;  and  Fig.  9  represents  a  valveless  pump,  of  which 
the  piston  oscillates  rapidly  up  and  down,  producing  an  alternating 
current  of  water  in  a  circuit  of  pipe.  The  electromotive  force  of 
the  alternator  A  not  only  has  to  overcome  the  resistance  of  the 
wire  in  order  to  cause  an  alternating  current  to  surge  back  and  forth 
through  the  circuit,  but  it  also  has  to  overcome  the  electrical  inertia 
of  the  circuit — first,  in  getting  a  pulse  of  current  started ;  and  second, 
in  stopping  this  pulse  of  current  and  starting  another  in  the  reverse 
direction.  The  pressure  developed  by  the  pump  P  not  only  has  to 


ALTERNATING-CURRENT  MACHINERY 


11 


overcome  the  frictional  resistance  of  the  pipe  in  order  to  cause  an 
alternating  current  of  water  to  surge  back  and  forth  through  the 
pipe,  but  it  also  has  to  overcome  the  inertia  of  the  water  in  the  pipe 

P/PE 


W/ffE 


Simple    Alternating    Circuit 


Fig.  9. 


Water  Analogy  for  an  Alter- 
nating Circuit 


— first,  in  getting  a  pulse  of  water  current  started;  and  second,  in 
stopping  this  pulse  of  water  current  and  starting  another  in  the 
reverse  direction. 

Fig.  10  represents  an  alternator  producing  alternating  current 
in  a  circuit  of  wire  which  contains  a  condenser  C;  and  Fig.  11  rep- 
resents a  valveless  pump  producing  an  alternating  current  of  water 
in  a  circuit  of  pipe,  which  leads  to  a  chamber  H  H,  across  which 
is  stretched  an  elastic  diaphragm  DD.  In  this  case  the  pressure 
developed  by  the  pump  has  to  overcome  the  frictional  resistance 
of  the  pipe,  the  inertia  of  the  water,  and  the  elastic  reaction  of  the 
diaphragm  DD.  Similarly  the  alternating  electromotive  force  of 

WIRE 


W/RE 


Fig.  10. 


Alternating  Circuit  Containing 
Condenser 


Fig.   11.     Water  Analogy  for  Alternating 
Circuit  with  Condenser 


the  alternator  A,  Fig.  10,  has  to  overcome  the  electrical  resistance 
of  the  wire,  the  electrical  inertia  of  the  wire  circuit,  and  the  electro- 
elastic  reaction  of  the  insulating  material,  or  dielectric,  between  the 
plates  of  the  condenser. 


12 


ALTERNATING-CURRENT  MACHINERY 


The  electrical  inertia  of  a  circuit  is  called  its  inductance  and 
the  electro-elasticity  of  a  co  idenser  is  called  its  capacity,  and  it  is  to 
inductance  and  capacity  that  the  peculiar  features  of  alternating- 
current  calculations  are  due. 

The  effect  of  capacity  is  strikingly  shown  by  the  fact  that  an 
alternating  current  may  be  made  to  flow  through  a  circuit  which 
for  direct  currents  would  be  an  open  circuit  like  that  shown  in  Fig. 
10.  Thus  an  alternator  connected  to  long  transmission  lines,  which 
are  disconnected  at  the  distant  end  and  perfectly  insulated  from  the 
ground,  will  send  a  considerable  alternating  current  into  the  lines, 
which  current  can  be  measured  by  an  alternating-current  ammeter. 
In  such  a  case  the  current  is  called  the  charging  current  of  the  line; 


.    POSITIVE  VALUES    ,  _ 

E/        OF    E.M.F.       \E 


I     AXIS   OF  TIME 


\|    NEGATIVE  VALUES/ 

M      OF  E.M.F.        /£ 


jEGATIVE  VALUES/ 

F   E.M.F.     / 


_J 


ONE CYJ^LE       OR  

TWO          ALTERNATIONS 


Fig.  12.     Development  of  Three  Field  Magnet  Poles  and  E.  M.  F.  Curve  for  One  Cycle 

and  it  may  amount  to  several  amperes,  according  to  the  length  of 
the  line,  the  distance  apart  and  size  of  wires,  and  the  electromotive 
force  of  the  alternator. 

Graphical  Representation  of  Alternating  Electromotive  Forces 
and  Currents.  When  an  armature  conductor  of  an  alternator  ap- 
proaches a  north  pole  of  the  field  magnet,  the  electromotive  force 
of  the  machine  rises  in  value  as  the  conductor  enters  the  strong 
field  under  the  pole;  and  the  electromotive  force  falls  in  value  as 
the  conductor  passes  from  under  the  pole.  As  the  conductor  passes 
the  point  midway  between  two  adjacent  poles,  the  electromotive 
force  of  the  machine  falls  to  zero,  since  no  lines  of  the  force  are  cut 


ALTERNATING-CURRENT  MACHINERY  13 

at  this  point.  As  the  armature  continues  to  revolve  and  the  con- 
ductor approaches  the  next  (the  south)  pole  of  the  field  magnet, 
the  electromotive  force  of  the  machine  again  increases  in  value,  but 
in  a  direction  opposite  to  that  of  the  previous  electromotive  force; 
and  it  falls  again  to  zero  as  the  conductor  passes  from  under  the 
south  pole  and  reaches  the  point  midway  between  the  next  pair  of 
poles. 

In  Fig.  12  are  represented  the  development  of  three  successive 
poles  S,  N,  S  of  the  field  magnet  of  an  alternator,  from  which  the 
lines  of  magnetic  flux  are  emanating,  and  spreading  out  more  or  less 
as  they  enter  the  armature  core.  The  armature  core,  also  a  developed 
view,  is  shown  as  having  only  one  slot  A,  which  contains  a  number 
of  armature  conductors.  The  ordinates  of  the  curve  E  E  E  E  E  E 
represent  the  successive  instantaneous  values  of  the  electromotive 
force  induced  in  the  armature  conductors  as  the  slot  moves  from 
left  to  right. 

The  duration  of  one 
cycle  is  indicated  in  the 
figure,  and  this  cycle  repeats 
itself  as  the  conductors  pass 
by  successive  pairs  of  field 
poles.  Thus,  in  a  ten-pole 
alternator,  there  would  be 
five  complete  cycles,  or  five 
complete  waves  of  the  elec- 
tromotive curve  for  each  Fig-13-  Typical.fe^;.(^^eeforAIternator~ 
revolution.  When  a  wave 
repeats  itself  after  a  definite  time  interval,  it  is  called  a  periodic  wave. 

The  curve  E  E  E  E  E  E  is  called  the  electromotive  curve  or  electro- 
motive force  wave  of  the  alternator. 

A  curve  of  which  the  ordinates  represent  the  successive  instan- 
taneous values  of  the  alternating  current  and  of  which  the  abscissas 
represent  time,  is  called  an  alternating-current  curve  or  alternating- 
current  wave. 

Figs.  13,  14,  and  15  show  typical  forms  of  electromotive  force 
curves  given  by  commercial  alternators.  Fig.  13  shows  what  is 
called  a  "peaked"  wave;  Fig.  14  shows  a  "flat-topped"  wave;  and 
Fig.  15  shows  a  "sine"  or  "sinusoidal"  wave.  All  three  waves  are  of 


14  ALTERNATING-CURRENT  MACHINERY 

course  periodic.  The  exact  shape  of  the  electromotive  force  wave 
given  by  an  alternator  depends  upon  the  relations  between  pole 
pitch  (distance  from  center  to  center  of  adjacent  poles),  width,  and 

shape  of  pole  faces,  width 
of  armature  coils,  and  dis- 
tribution of  coils  on  the  ar- 
W  mature.  Alternators  which 

give  electromotive  force 
waves  approximating  a  sine 

Fig.  14.     Typical  E.  M.  F.  Curve  for  Alternator —  „ 

"Fiat  Topped"  Wave  wave,     are     preterrcd     lor 

power  transmission. 

Average  and  Effective  Values  of  E.  M.  F.  The  average  value 
of  an  alternating  electromotive  force  or  current  during  a  complete 
cycle  is  zero,  inasmuch  as  similar  sets  of  positive  and  negative  values 
occur. 

The  average  value  of  an  electromotive  force  or  current  during 
the  positive  (or  negative)  part  of  a  cycle  is  usually  spoken  of  briefly  as 
the  "average  value"  or  "mean  value,"  and  is  not  zero. 

Consider  now  an  alternating  current,  of  which  the  instantaneous 
value  is  i.  The  rate  at  which  heat  is  generated  in  a  circuit  through 
which  the  current  flows  is  i2R}  where  R  is  the  resistance  of  the  circuit; 
and  the  average  rate  at  which  heat  is  generated  in  the  circuit  is  R 
multiplied  by  the  average  value  of  i2. 

A  continuous  current  which  would  produce  the  same  heating 
effect  would  be  one  of  which  the  square  is  equal  to  the  average  value 
of  i2,  or  of  which  the  actual  value  is  equal  to  V  average  i2.  This 
square  root  of  the  average  square  of  an  alternating  current  is  called 
the  effective  value  of  the  alternating  current.  Similarly,  the  square 

root  of  the  average  square 
of  an  alternating  electro- 
motive force  is  called  the 
effective  value  of  the  alter- 
nating electromotive  force. 

Fig.   15.     Typical  E.  M.  F.  Curve  for  Alternator  —         Voltmeters  and  ammetei'S 

"Sine"  Wave 

used  for  measuring  alter- 
nating electromotive  force  or  current  always  give  effective  values 
irrespective  of  wave  form;  and  in  specifying  an  alternating  elec- 
tromotive force  or  current,  its  effective  value  is  always  used. 


ALTERNATING-CURRENT  MACHINERY 


15 


Example.  Ten  successive  instantaneous  values  of  an  alternating  elec- 
tromotive force  during  half  a  cycle  are  0,  30,  60,  80,  90,  95,  90,  80,  60,  and 
30  volts.  The  sum  of  these  values  is  615  volts,  which,  divided  by  the  num- 
ber of  values,  namely  ten,  gives  61.5  volts,  which  is  the  average  value  of  this 
electromotive  force  during  half  a 

CYCLE-*)' 


E.M.F. 


Fig.  16.     Rectangular  Form  of  E.  M.  F.  Curve 


cycle- 

Squaring  each  of  the  above 

values,  adding  the  squares  together, 
and  dividing  their  sum  by  their 
number,  namely  ten,  gives  the 
average  value  of  the  square  of  the 

electromotive  force,  which  is  4,702.5  volts2;  and  the  square  root  of  this  aver- 
age square  is  68.57  volts,  which  is  the  effective  value  of  the  given  electro- 
motive force. 

Form  Factor.  The  ratio  effective  value  -r-  average  value,  depends 
upon  the  shape  of  the  electromotive  force  wave,  and  is  called  the 
"form  factor"  of  the  wave. 


Example.     The  form  factor  in  the  above  case  is 


or  1.115.  The 


68.57 

61.5 

form  factor  of  the  electromotive  force  curve  given  in  Fig.  15,  which  is  a  sine 
wave,  is  1.11.  The  more  peaked  the  wave  the  greater  the  value  of  its  form 
factor.  The  rectangular  electromotive  force  shown  in  Fig.  16  has  a  form 
factor  equal  to  unity,  which  is  the  least  possible  value  of  the  form  factor. 
This  rectangular  wave,  however,  is  never  realized  in  commercial  alternators. 

Instantaneous  and  Average  Power.  Let  e  be  the  value,  at  a  given 
instant,  of  the  electromotive  force  of  an  alternator  and  let  i  be  the 
value  of  the  current  at  the  same  instant.  Then  ei  is  the  power  in 
watts  which  is  delivered  by  the  alternator  at  the  given  instant; 
and  the  average  value  of  ei  during  a  complete  cycle  is  the  average 
power  delivered  by  the  alternator. 


Fig.  17.     E.  M.  F.,  -Current,  and  Power  Curves  for  an 
Alternator  in  a  Circuit  Containing  Inductance 

In  Fig.  17  the  full-line  curve  represents  the  electromotive 
force  of  an  alternator  and  the  heavy-dotted  curve  represents  the 
current  delivered  by  the  alternator  to  a  receiving  circuit  having 


16 


ALTERNATING-CURRENT  MACHINERY 


inductance,  such  as  an  induction  motor,  for  instance.  The  ordinates 
of  the  light-dotted  curve  represent  the  successive  instantaneous 
values  of  the  power  ei.  As  shown  in  the  figure,  the  power  has  both 


Fig.  18. 


E.  M.  F.,  Current,  and  Power  Curves  for  Alternating  Circuit  with 
Large  Inductance 


positive  and  negative  values;  the  alternator  does  work  on  the  cir- 
cuit when  ei  is  positive,  or  is  above  the  horizontal  axis  of  time;  and 
the  circuit  returns  power  to  the  alternator  when  ei  is  negative,  or 
is  below  the  horizontal  axis  of  time;  and  this  means  of  course,  that 
while  ei  is  negative,  the  dynamo  is  momentarily  a  motor  and  will 
be  for  the  moment  returning  power  to  the  fly  wheel  of  the  driving 
engine  or  turbine. 

When  the  inductance  of  the  receiving  circuit  is  very  large,  the 
electromotive  force  and  current  curves  are  related  as  shown  in  Fig. 
18;  the  instantaneous  power  ei  passes  through  approximately  similar 
sets  of  positive  and  negative  values,  as  shown  by  the  light-dotted 

curve;  and  the  average  power  is  approx- 
imately zero.  This  case  would  be  very 
closely  exemplified  by  an  alternator  con- 
nected to  a  transformer  whose  secondary 
was  open-circuited,  that  is,  supplying  no 
current. 

Harmonic  Electromotive  Forces  and 
Currents.  A  line  OP,  Fig.  19,  revolves, 
at  a  uniform  rate,/  revolutions  per  second 
about  a  point  0,  in  the  direction  of  the 
arrow  gh.  Since  the  length  of  OP  is  fixed, 
the  path  or  locus  of  the  point  P  will  be  a 
circle  about  0  as  a  center.  Consider  the 
projection  Ob  of  this  rotating  line  upon 
the  fixed  line  AB,  this  projection  being 
considered  positive  when  above  0  and  negative  when  below  0. 


Fig.  19. 


Diagram  of  Harmonic 
E.  M.  F.s 


ALTERNATING-CURRENT  MACHINERY  17 

A  harmonic  electromotive  force  (or  current)  is  an  ekctromotive 
force  (or  current)  which  is  at  each  instant  proportional  to  the  line  Ob. 

The  line  Ob  represents  at  each  instant  the  actual  value  e  of  the 
harmonic  electromotive  force  to  a  definite  scale,  and  the  length  of 
the  line  OP,  which  is  the  maximum  length  of  Ob,  represents  the 
maximum  value  E  of  the  harmonic  electromotive  force  to  the  same 
scale.  The  line  Ob  passes  through  a  complete  cycle  of  values  during 
one  revolution  of  OP,  and  so  also  does  the  harmonic  electromotive 
force  e.  Therefore,  the  revolutions  per  second  /  of  the  line  OP  is 
the  frequency  of  the  harmonic  electromotive  force  e.  The  rotating 
lines  E  and  I,  Fig.  20,  of  which  the  projections  on  a  fixed  line  (not 
shown  in  the  figure)  represent  the  actual  instantaneous  values  e 
and  i  of  a  harmonic  electromotive  force  and  a  harmonic  current, 
are  said  to  "represent"  the  harmonic  electromotive  force  and  cur- 
rent, respectively.  Of  course,  the  rotation  of  the  lines  E  and  I  is 
a  thing  merely  to  be  imagined.  The  rotation  is  understood  to  be  in 
a  counter-clockwise  direction,  as  indicated  in  Fig.  19. 

Clock  Diagram  Representation.  A  diagram 
in  which  a  number  of  electromotive  forces  or 
currents,  or  both,  are  represented  by  lines  im- 
agined to  be  revolving,  is  called  a  clock  diagram. 
Simple  problems  involving  relations  between  a 
number  of  harmonic  electromotive  forces  and 
currents  of  the  same  frequency ,  are  most  easily 
treated  by  means  of  the  clock  diagram.  The  ~ig>  20.  Ciock  Diagram 
proper  representation  of  alternating  electro- 
motive forces  and  currents  in  a  clock  diagram,  requires  that 

(a)  The  given  electromotive  forces  and  currents  be  harmonic,  and  be  of 
the  same  frequency. 

(b)  The  lengths  of  the  lines  represent  their  maximum  value  to  a  suitable 
scale  although  the  scales  chosen  for  volts  and  for  amperes  may  be 
different. 

(c)  The  direction  of  the  electromotive  forces  and  currents  be  indicated  by 
arrow  heads. 

(d)  The  relative  position  or  phase  of  the  electromotive  forces  and  currents 
be  constant  and  indicated  by  the  angle  between  the  various  lines  rep- 
resenting the  given  quantities. 

When  an  electromotive  force  (or  current)  wave  is  not  a  sine 
curve,  the  electromotive  force  (or  current)  is  not  harmonic,  and  can- 


18 


ALTERNATING-CURRENT  MACHINERY 


not  properly  be  represented  by  a  line  in  a  clock  diagram,  because 
the  projection  of  the  rotating  line  is  not  at  each  instant  proportional 
to  the  electromotive  force  (or  current).  The  approximate  represen- 


270° 

Fig.  21.     Method  of  Plotting  Sine  E.  M.  F.  Curve 

tation,  by  lines  in  a  clock  diagram,  of  non-harmonic  electromotive 
forces  (or  currents) — such,  for  example,  as  those  represented  by  the 
curves  in  Figs.  13,  14,  and  16,  depends  upon  the  finding  of  harmonic 
electromotive  forces  (or  currents)  which  for  the  particular  purpose 
in  view  are  approximately  equivalent  to  the  actual  given  electro- 
motive forces  (or  currents). 

Graphical  Representation.  A  harmonic  electromotive  force  or 
current  is  represented  by  a  sine  wave  as  shown  in  Fig.  15.  The  rela- 
tion between  the  rotating  line  OP  in  Fig.  19  and  the  sine-wave  curve 
of  electromotive  force  is  shown  as  follows : 

Divide  the  circumference  of  the 
circle  in  Fig.  21  into  equal  parts, 
and  lay  off  a  horizontal  line  divided 
into  the  same  number  of  equal  parts. 

Draw  horizontal  dotted  lines 
through  each  division  on  the  cir- 
cumference of  the  circle,  and  vertical 
dotted  lines  through  the  correspond- 
ing divisions  on  the  horizonta"  line. 
The  points  of  the  intersection  of 
these  pairs  of  dotted  lines  are  points 
on  a  curve  which  is  a  curve  of  sines. 

A  flat  loop  of  wire  with  its,ter- 
minals  connected  to  two  collect- 
ing rings  gives  a  harmonic  elec- 
tromotive force  when  it  is  rotated  at  constant  speed  in  a  uniform 
magnetic  field.    This  arrangement  is  shown  in  Fig.  22. 


Fig.  22,     Simple  Dynamo  Diagram 


ALTERNATING-CURRENT  MACHINERY  19 

Algebraic  Representation.  The  line  OP,  Fig.  19,  revolves  uni- 
formly /  revolutions  per  second  and,  therefore,  it  turns  through  27rf 
radians*  per  second,  since  there  are  2n  radians  in  a  revolution;  that  is 

W=27T/  (8) 

in  which  aj  is  the  angular  velocity  of  the  line  OP  in  radians  per 
second.  Let  time  be  reckoned  from  the  instant  that  OP  coincides 
with  Oa;  then,  after  t  seconds,  OP  will  have  turned  through  the 
angle  /?  (=^t);  and  from  Fig.  19  we  have 

Ob  =  OP  sin  /?  =  OP  sin  cut 

since  Ob  is  the  projection  of  OP  on  the  line  AB.  But  Ob  represents 
the  actual  value  e  of  the  harmonic  electromotive  force  at  the  time 
t,  and  OP  represents  its  maximum  value  E;  therefore 

e  =  E  sin  ut  (9) 

is  an  algebraic  expression  for  the  actual  value  e  of  a  harmonic  electro- 
motive force  at  time  t,  E  being  the  maximum  value  of  e,  and— 
being  the  frequency  according  to  equation  (8). 

Similarly  ,  .  /ln. 

i  =  I  sm  cot  (10) 

is  an  algebraic  expression  for  the  actual  value  i  of  a  harmonic  cur- 
rent at  time  t,  I  being  the  maximum  value  of  i. 

If  time  is  reckoned  from  the  instant  that  OP,  Fig.  19,  coincides 
with  the  line  06,  then  equations  (9)  and  (10)  become 

e  =  E  cos  cot 
i=I  cos  cot 

Synchronism.  Two  alternating  electromotive  forces  or  currents 
are  said  to  be  in  synchronism  when  they  have  the  same  frequency. 
Two  alternators  are  said  to  run  in  synchronism  when  their  electro- 
motive forces  and  frequencies  are  similar. 

Phase  Difference.  Consider  two  harmonic  electromotive  forces 
represented  by  the  ordinates  of  the  curves  EI  and  Ev  Fig.  23.  The 
electromotive  force  represented  by  the  curve  Et  reaches  its  maximum 
value  before  the  electromotive  force  represented  by  the  curve  Er 
The  electromotive  force  EI  is  said  to  lead,  or  to  be  ahead  of,  the 


*The  unit  of  angle  chiefly  used  in  mechanics  and  in  all  theoretical  work  is  the  radian. 
It  is  the  angle  of  which  the  arc  is  numerically  equal  to  the  radius  (of  a  circle).  There  are, 
therefore,  27T  radians  in  one  circumference. 


20 


ALTERNATING-CURRENT  MACHINERY 


electromotive  force  E2  in  phase.   Conversely,  the  electromotive  force 
E2  is  said   to   lag    behind  or  to    follow   the   electromotive  force 

EI  in  phase.    The  same  two 

Ei^xpv T-.ES  electromotive  forces  El  and  E2 

are  also  represented  by  the 
lines  OE^  and  OE2  in  the  clock 
diagram,  Fig.  24.  Here  the 
line  OE2  is  behind  OElt  since 
the  imagined  rotation  about 
0  as  a  center  is  counter-clock- 


TIME 


Fig.  23.     Curves  of  Two  Related  E.  M.  F.s 


wse. 


The  phase  difference  is  the  time  interval  6  in  Fig.  23,  or  the  angle 
0  between  OEl  and  OE2  in  Fig.  24.  If  according  to  equation  (9)  the 
actual  value  of  the  harmonic  electromotive  force  .Eiis  e\—E\  sin  cot, 
then  the  actual  value  of  the  electromotive  force  E2t  which  lags  0  de- 
grees behind  Elf  is  e2=E2  sin  (cot  —  6).  Similarly,  if  E2  were  taken 
as  the  reference  line  in  the  diagram,  its  actual  value  would  be  e2 
=  E2  sin  cot,  and  the  value  of  EI  would  then  be  e\  —  E\  sin  (cot-{-0). 
When  the  angle  Q,  Fig.  24,  is  zero,  as  shown  in  Fig.  25,  the 
electromotive  forces  El  and  E2  are  said  to  be  in  phase.  In  this  case 
the  electromotive  forces  increase  together  and  decrease  together; 


Fig.  24.     Clock  Diagram  of 
E.  M.  F.s  for  Fig.  23fl 


Fig.  25.     Clock  Diagram  for 

Curves  in  Fig.  23  with 

E.  M.  F.s  in  Phase 


that  is,  when  El  is  zero,  E2  is  also  zero ;  and  when  E1  is  at  its  maximum 
value,  so  also  is  E2>  etc.  Therefore,  el  =  El  sin  cot  and  e2=  E2  sin  cot. 
When  6  =90°,  as  shown  in  Fig.  26,  the  two  electromotive  forces 
are  said  to  be  in  quadrature.  In  this  case  one  electromotive  force  is  zero 
when  the  other  is  a  maximum,  etc.,  or  e1=E1  sin  cot  and  e2=E2sin 

H-f). 


ALTERNATING-CURRENT  MACHINERY 


21 


Fig.  26. 
E.  M.  F.s  of  Fig.  23    / 


Diagram  of 

s  of  Fig.  f  ' 
in  Quadrature 


When  6  =  180°,  as  shown  in  Fig.  27,  the  two  electromotive 
forces  are  said  to  be  in  opposition.  In  this  case  they  are  at  each 
instant  opposite  in  sign;  and  when  one  is  at  its  positive  maximum, 
the  other  is  at  its  negative  maximum,  etc.  In 
this  case  #1  =  EI  sin  cot  and  e2=  E2  sin  (cot  ±  TT). 
It  is  to  be  particularly  noted  that  the  principle 
of  phase  difference  which  has  been  illustrated 
in  Figs.  22-25  for  the  case  of  two  harmonic 
electromotive  forces  EI  and  E2}  applies  equally 
to  the  case  of  two  harmonic  currents  /i  and 
72  and  to  the  case  of  an  electromotive  force 
EI  and  a  current  /i.  Thus  if  in  the  clock 
diagram,  Fig.  20,  E  represented  a  harmonic 
electromotive  force  having  a  maximum  value 
of  1,000  volts,  and  I  represented  a  harmonic 
current  having  a  maximum  value  of  10  amperes  with  a  phase  dif- 
ference of  0=30°  between  them,  the  instantaneous  values  of  E  and 

/  would  be 

e  =  1000  sin  cot 

i=     10  sin  '(cot  — «-) 

Addition  of  Harmonic  Electromotive 
Forces  and  Currents.  Consider  two  har- 
monic electromotive  forces  of  which  the 
successive  instantaneous  values  e±  and  e2  are  represented  by  the  pro- 
jections of  the  lines  EI  and  E2)  Fig.  28,  which  are  imagined  to  be 
revolving  about  the  point  0.  These  elec- 
tromotive forces  being  of  the  same  fre- 
quency, the  lines  EI  and  E2  revolve  at 
the  same  speed,  so  that  the  angle  between 
EI  and  E2  remains  unchanged  in  value. 
The  ordinary  arithmetical  sum  of  e±  and 
e2,  namely  e±  -f-  e2,  is  a  harmonic  electro- 
motive force  of  the  same  frequency  as  e\  and 
e2-y  and  this  electromotive  force  (ei-\-e2)  is  represented  by  the  projection 
of  the  line  E,  Fig.  28,  which  revolves  at  the  same  speed  as  EI  and  E2. 
Ths  is  evident  when  we  consider  that  the  projection,  on  any 
line,  of  the  diagonal  of  a  parallelogram  is  equal  to  the  sum  of  the 


Fig.  27 


Diagram  of  E.  M.  F.s  of 
Fig.  23  When  in  Opposition 


Fig.  28.     Vector  Diagram  Show- 
ing Two  Harmonic  E.  M.  F.s 


22 


ALTERNATING-CURRENT  MACHINERY 


Fig.  29.     Diagram  Showing  Addition  of 
Harmonic  E.  M.  F.s 


projections  of  two  adjacent  sides  of  the  parallelogram,  as  shown  in 
Fig.  29.  The  projection  of  El  is  Oc,  which  represents  ej  and  the 

projection  of  E2  is  equal  to  cd, 
which  represents  e2.  The  projec- 
tion of  the  diagonal  Ob  of  the 
parallelogram  is  Od,  which  is  the 
sum  of  Oc  and  cd.  The  two  lines 
marked  E2  in  Fig.  29  are  equal  and 
parallel  and  have,  therefore,  the 
same  projected  length  on  the  ver- 
tical line. 

As  a  corollary  to  the  above,  it 
may  be  stated  that  the  ordinary 
arithmetical  sum  (e.  +  £0  +  e  + 

123 

etc.)  of  the  instantaneous  values 

of  any  number  of  harmonic  electromotive  forces  (or  currents)  is 
another  harmonic  electromotive  force  (or  current)  of  the  same  fre- 
quency; it  is  represented  in  magnitude  and  phase  by  a  line  that  is 
the  geometric  (or  vector)  sum  of  the  lines  representing  the  given 
individual  electromotive  forces  (or  currents).  This  is  evident  when 
we  consider  that  e1-}-e2  is  a  harmonic  electromotive  force  (or  current) 
according  to  the  above  discussion;  and  this,  added  to  e3,  gives  an 
electromotive  force  (or  current)  which  is  harmonic  and  of  the  same 

frequency  as  el9  e2,  and  e3. 

The  geometric,  or  vector,  sum  of  a  num- 
ber of  lines  is  obtained  as  follows :    Given 
three  lines  OE'l9  OE2,  and  OE3,  Fig.  30. 
Find  the  diagonal  OA  of  the  parallelo- 
gram constructed  on  OE±  and  OE2  as  sides. 
This  gives  the  vector  sum  of  OE1  and  OE2. 
Next  construct  a  parallelogram  on  OA  and 
OE3  as  sides;  the  diagonal  OE  of  this  par- 
allelogram is  the  vector  sum  of  the  three 
given  lines.    This  line  OE  is  the  closing 
side  of  the  polygon  formed  by  drawing  OElf  then  drawing  OE2  from  the 
extremity  of  OEl  (this  giving  the  point  A),  and  then  drawing  OE3 
from  A.    This  method  is  called  addition  by  means  of  the  vector  polygon. 
For  example,  two  alternators  A  and  B  running  in  synchronism 


Fig.  30.     Addition  of  E.  M.  F.s 
by  Vector  Polygon 


ALTERNATING-CURRENT  MACHINERY 


23 


are  connected  in  series  between  mains  as  shown  in  Fig.  31.  If  the 
electromotive  forces  of  A  and  B  are  in  phase,  the  electromotive 
force  between  the  mains  will  be  simply  the  numerical  sum  of  the 
electromotive  forces  of  A  and  B.  If,  on  the  other  hand,  the  electro- 
motive forces  of  A  and  B  differ  in  phase,  the  state  of  affairs  will  be 
as  represented  in  Fig.  32,  in  which  the  lines  A  and  B  represent  the 


mom 


man 


Fig.  31.     Two  Alterna- 
tors Running  in  Syn- 
chronism 


Fig.  32,     Vector  Diagram  of  Con- 
ditions in  Fig.  31  if  E.  M.  F.a 
Differ  in  Phase 


electromotive  forces  of  the  alternators  A  and  B,  respectively,  0  is 
the  phase  difference  of  A  and  B,  and  the  line  E  represents  the  electro- 
motive force  between  the  mains.  The  line  E  is  the  vector  sum  or 
resultant  of  A  and  B,  and  as  shown  is  the  diagonal  of  a  parallelo- 
gram constructed  on  A  and  B  as  sides. 

Again,  alternators  A  and  B  running  in  synchronism  are  con- 
nected in  parallel  between  the  mains  as  shown  in  Fig.  33.  Let  the 
lines  A  and  B,  Fig.  34,  represent  the  currents  given  by  the  alter- 
nators A  and  B,  respectively,  the  phase  difference  between  the  cur- 
rents being  0;  then  the  current  in  the  main  line  is  represented  by  /. 

Consider  the  case  of  two  circuits  A  and  B,  Fig.  35,  connected  in 


mo/n 


mom 


Fig.  33. 


Two  Alternators  Running  in  Synchronism 
Connected  in  Parallel 


Fig.    34.     Vector  Diagram  of  Currents 
for  the  Condition  of  Fig.  33 


series  between  the  mains  of   an  alternator.     The  line  E,  Fig.  36, 
represents  the  electromotive  force  between  the  mains;  the  line  A 


24 


ALTERNATING-CURRENT  MACHINERY 


represents  the  electromotive  force  between  the  terminals  of  the 
circuit  A;  and  the  line  B  represents  the  electromotive  force  be- 
tween the  terminals  of  the  circuit  B.  The  circuits  A  and  B  are 


ma/n 


mam 


-•* 


Fig.  35.     Diagram  of  Two  Coils 
in  Series  with  an  Alternator 


Fig.  36.    Diagram  of  E.  M.  F.s  for 
Conditions  Shown  in  Fig.  35 


supposed  to  have  inductance.  If  either  of  the  circuits  contains  a 
condenser,  then  the  electromotive  forces  A  and  B,  Fig.  36,  may  be 
nearly  opposite  to  each  other  in  phase,  and  A  and  B  may  each  be 
indefinitely  greater  than  the  electromotive  force  E  between  the 
mains. 

Again,  two  circuits  A  and  B,  Fig.  37,  are  connected  in  parallel 
across  the  terminals  of  an  alternator  as  shown.  The  current  /  from 
the  alternator  is  related  to  the  currents  A  and  B  as  shown  in  Fig.  38. 
If  either  of  the  circuits  A  or  B  contains  a  condenser,  then  the  cur- 
rents A  and  B  may  be  nearly  opposite  to  each  other  in  phase,  and 
the  currents  A  and  B  may  each  be  indefinitely  greater  than  the 

current  I  from  the  alternator. 

Subtraction  of  Harmonic  Electromotive 
Forces  and  Currents.  One  harmonic  elec- 
tromotive force  (or  current)  is  subtracted 
from  another  by  reversing  the  direction  of 
the  line  that  represents  it  in  the  clock  dia- 
gram, and  then  adding  the  reversed  line  (or 
vector)  to  the  other.  An  example  of  the 
subtraction  of  harmonic  electromotive  forces 

Fig.   37.     Coils  in  Parallel  with       .„  ,  .  ,.  .,,     .,        ,. 

an  Alternator  will  be  given  in  connection  with  the  discus- 

sion of  three-phase  electromotive  force. 

Relation  between  Maximum  and  Effective  Values.  The  effective 
value  E  or  I  of  a  harmonic,  electromotive  force,  or  current — that  is, 


main 


ALTERNATING-CURRENT  MACHINERY 


25 


one  whose  graph  is  a  sine  wave — is  equal  to  the  maximum  value  E 
or  I  divided  by  the  square  root  of  2.    That  is 


E 


1/2 


(11) 


(12) 


in  Fig.  37 


1/2 

This  may  be  shown  as  follows:  Let  e 
(=E  sin  tot)  be  a  harmonic  electromotive 
force.  To  find  the  average  value  of  e2 
(=  E2  sin2  tot],  it  is  necessary  to  find  the  aver- 
age value  of  the  square  of  the  sine  of  the 
uniformly  variable  angle  tot.  We  have  the 
general  relation 

(a)  sin2  tot  +  cos2  tot  =  1 
so  that 

(b)  Av.  sin2  tot  +  Av.  cos2  tot  =  1 

Now,  during  a  cycle,  the  cosine  of  a  uniformly  variable  angle  passes 
similarly  through  the  same  set  of  values  as  the  sine;  hence  Av. 
sin2  tot  and  Av.  cos2  tot  are  equal,  so  that  from  equation  (b)  above,  we 
have 

2  Av.  sin2  tot  =  1 


or 


Av. 


The  average  value  of  e2  is 

Av.  e2  =  E2  Av.  sin2  tot 


or 


and 


In  Fig.  19  the  length  of  the  revolving  line  OP  was  understood 
to  represent  the  maximum  value  of  the  harmonic  electromotive 
force  (or  current).  When,  however,  a  number  of  harmonic  electro- 
motive forces  (or  currents)  are  represented  to  scale  by  lines  in  a 
clock  diagram,  the  lengths  of  the  lines  may  be  interpreted  as  giving 


26  ALTERNATING-CURRENT  MACHINERY 

not  maximum  but  effective  values,  since  there  is  a  constant  ratio 
V  2  between  the  effective  and  the  maximum  values  of  each  of  the 
electromotive  forces  (or  currents)  represented  in  the  diagram. 

NOTE.  It  is  desirable  to  interpret  the  lines  in  a  clock  diagram  in  terms 
of  effective  values  rather  than  maximum  values,  because  effective  values  are 
always  given  by  measuring  instruments  and  are  nearly  always  used  in  nu- 
merical calculation.  Therefore,  unless  it  is  expressly  stated  to  the  contrary, 
the  lines  in  clock  diagrams  are  always  understood  to  represent  effective  values. 

For  example,  a  certain  harmonic  alternating  current  gives  a 
reading  of  100  amperes  on  an  alternating-current  ammeter,  and  its 
effective  value  is,  therefore,  100  amperes.     This  harmonic  current 
actually  pulsates   between   zero  and  a  maximum  value  of  ±i/  2 
X  100  amperes,  or  ±  141.4  amperes. 

Again,  a  certain  harmonic  alternating  electromotive  force  gives  a 
reading  of  1,000  volts  on  an  alternating-current  voltmeter,  and  its 
effective  value  is,  therefore  1,000  volts.  This  electromotive  force 
actually  varies  between  zero  and  a  maximum  value  of  ±  V  2  X 
1,000  volts,  or  ±  1.414  volts. 

The  above  simple  relation  between  'maximum  and  effective 
values  is  true  only  for  harmonic  (that  is,  sine-wave)  electromotive 
forces  and  currents.  In  general,  the  maximum  values  of  alternating 
electromotive  forces  or  currents  cannot  be  inferred  from  effective 
values  as  measured  by  voltmeters  or  ammeters.  Thus,  an  alter- 
nating electromotive  force  which  is  known  to  have  a  peaked-wave 
form  might  have  a  maximum  value  very  greatly  in  excess  of  V  2 
times  its  effective  value. 

Expression  for  Power,  (a)  When  the  current  is  in  phase  with 
the  electromotive  force,  that  is,  when  the  circuit  is  non-inductive, 
then  the  power  (average  ei,  see  page  15)  is 

P=EI  (13) 

in  which  P  is  the  power  in  watts,  E  is  the  effective  value  of  the 
electromotive  force  in  volts,  and  /  is  the  effective  value  of  the  cur- 
rent in  amperes.  Equation  (13)  is  identical  with  the  power  equation 
for  direct-current  circuits. 

(b)  If  the  phase  difference  between  current  and  electromotive 
force  in  a  given  circuit  were  90  degrees,  which  can  never  actually 
occur,  then  the  power  (average  value  of  ei)  would  be  equal  to  zero, 
as  explained  on  page  16. 


ALTERNATING-CURRENT  MACHINERY  27 

(c)  When  the  phase  difference  between  current  and  electro- 
motive force  is  0°,  as  shown  in  Fig.  39,  then 

P=  El  cos  6  (14) 

in  which  P  is  the  power  (average  ei)  in  watts,  E  is  the  effective  value 
of  the  electromotive  force  in  volts,  and  /  is  the  effective  value  of 
the  current  in  amperes. 

For  example,  the  given  current  I  shown  in  Fig.  39  may  be 
thought  of  as  resolved  into  two  components,  as  shown  in  Fig.  40. 
One  of  these  components  7  cos  6  is  parallel  to  (that  is,  in  phase  with) 
E;  and  the  other  7  sin  6  is  at  right  angles  to  E.  The  power  corre- 
sponding to  the  actual  current  I  may  be  thought  of  as  the  sum  of 
the  powers  corresponding  to  its  two  components,  respectively.  But 
the  component  I  sin  6  is  at  right  angles  to  E,  as  in  (b)  above;  hence 
the  power  corresponding  to  it  is  zero.  This  component  is,  therefore, 
frequently  called  the  wattless  component  or  better  the  reactive  com- 
ponent of  the  given  current. 

On  the  other  hand,  the  com- 
ponent I  cos  0  is  parallel  to  (that 
is,  in  phase  with)  E,  as  in  (a) 
above;  hence  the  power  corre- 
sponding to  this  component  is 
equal  to  EX  I  cos  6.  The  com- 
ponent /  cos  0  of  the  given  cur- 
rent /  is  frequently  Called  the  FiS-  39-  Diagram  Shying  Phase  Difference 

Between  E.  M.  F.  and  Current 

power  component  of   7;  and  the 

factor  cos  6  is  called  the  power  factor  of  the  circuit. 

Inductance.  It  has  been  pointed  out,  page  11,  that  an  electric 
circuit  has  a  certain  kind  of  inertia  analogous  to  the  inertia  of  water 
in  a  circuit  of  pipe,  and  it  was  there  noted  that  this  inertia  of  an 
electric  circuit  is  called  inductance.  If  an  electric  current  in  a  cir- 
cuit is  made  to  change  in  value,  a  portion  of  the  electromotive  force 
acting  upon  the  circuit  must  be  used  to  cause  the  current  to  change. 

In  the  same  way  a  force  over  and  above  that  required  to  over- 
come frictional  resistance  must  act  upon  a  moving  body  to  accelerate 
it,  that  is,  to  make  its  speed  increase.  The  inertia  of  a  body  is  meas- 
ured by  the  force  required  to  accelerate  it  at  the  rate  of  unit  change 
in  speed  per  second;  and  the  inductance  of  a  circuit  is  measured  by 
the  electromotive  force  required  to  cause  a  current  in  the  circuit 


28 


ALTERNATING-CURRENT  MACHINERY 


to  change  at  the  rate  of  one  ampere  per  second.  A  circuit  is  said  to 
have  an  inductance  of  one  henry*  when  one  volt  (over  and  above 
the  electromotive  force  required  to  overcome  the  electrical  resist- 
ance) will  cause  the  current  to  change  at  the  rate  of  one  ampere  per 
second. 

Let  x  be  the  rate  in  amperes  per  second  at  which  the  current 
in  a  circuit  is  increasing  in  value.  Then  the  electromotive  force  E 
(over  and  above  that  required  to  overcome  the  resistance  of  the 
circuit)  required  to  cause  the  current  to  increase  at  this  rate  is 


E=Lx 


(15) 


in  which  L  is  the  inductance  of  the  circuit  in  henrys,  E  being  ex- 
pressed in  volts. 

Example.  The  coil  of  a  certain 
large  electromagnet  has  2.5  henrys  of 
inductance  and  5  ohms  of  resistance. 
At  a  given  instant  this  coil  is  connected 
to  110- volt  direct-current  mains.  At 
the  instant  of  connecting  the  coil,  the 
current  is  zero,  and  all  of  the  110  volts 
is  used  to  cause  the  current  in  the 
coil  to  increase,  so  that,  according  to 

E 

equation  (15),     x   is  equal    to  — 7-, or 


Fig.  40. 


Resolution  of  the  I  of  Fig.  39  into 
Two   Components 


110  volts  -s-  2.5  henrys,  or  44  amperes 
per  second.  That  is,  the  current  in  the 
magnet  coil  begins  to  increase  at  the  rate 
of  44  amperes  per  second.  When  the 

current  in  the  magnet  coil  has  reached  the  value  of  10  amperes,  50  volts  ( =  5 
ohms  X 10  amperes)  of  the  total  110  volts  are  used  in  overcoming  the  resist- 
ance of  the  coil,  so  that  60  volts  ( =  110  volts  — 50  volts)  are  used  to  make  the 
current  increase.  Therefore,  as  the  current  in  the  coil  passes  the  value 


E 


,  or  60  volts  -5-  2.5  henrys,  or  24 


of  10  amperes,  it  is  increasing  at  the  rate 
amperes  per  second. 

The  inductance  of  a  coil  wound  on  a  given  spool  is  proportional 
to  the  square  of  the  number  of  turns  N  of  wire.  For  example,  a 
given  spool  wound  with  No.  16  wire  has  500  turns  and  an  induc- 
tance of,  say,  0.0025  henry;  the  same  spool  wound  with  No.  28 
wire  would  have  about  eight  times  as  many  turns,  and  its  induc- 
tance would  be  about  64  times  as  great,  or  0.16  henry. 


*The   henry  is   a   very  large   inductance,   and  the  inductances   usually   met  with  in 
practice  are  expressed  in  thousandths  of  a  henry,  that  is,  in  milli-henrys. 


ALTERNATING-CURRENT  MACHINERY  29 

The  inductance  of  a  coil  of  given  shape  is  proportional  to  its 
linear  dimensions,  the  number  of  turns  of  wire  being  unchanged. 
For  example,  a  given  coil  has  an  inductance  of  0.022  henry;  and  a 
coil  three  times  as  large  in  length,  diameter,  etc.,  but  having  the  same 
number  of  turns  of  wire,  has  an  inductance  of  0.066  henry. 

Formulas  for  Inductance.  The  inductance  in  henrys  of  a  coil 
of  wire  wound  in  a  thin  layer  on  a  long  wooden  cylinder  of  length 
/  centimeters  and  of  radius  r  centimeters,  is 

47r2  r2  N* 


in  which  N  is  the  total  number  of  turns  of  wire  in  the  coil.  This 
equation  is  strictly  true  for  very  long  coils  wound  in  a  thin  layer; 
but  the  equation  is  also  very  useful  in  calculating  the  approxi- 
mate inductance  of  even  short,  thick  coils.  Thus,  a  coil  25  centi- 
meters long  and  2|  centimeters  mean  radius,  containing  150  turns 
of  wire,  has  an  approximate  inductance  of 

L=4^(2.5)*X150*=0000  '-.. 

25  X  109 

If  a  coil  of  N  turns  of  wire  is  wound  on  a  long  rod  of  iron,  in- 
stead of  wood  or  other  non-magnetic  material,  the  inductance  in 
henrvs  is  given  by  the  equation 


JX10- 

in  which  L,  r,  N,  and  /  are  the  same  as  in  equation  (16),  and  t*.  is  the 
permeability  of  the  iron  core  at  the  particular  flux  density  produced 
in  the  iron.  This  equation  applies  to  any  iron  rod  of  length  /  centi- 
meters and  radius  r  centimeters,  wound  with  N  turns  of  wire,  whether 
in  the  form  of  a  long,  straight  rod  or  bent  into  a  closed  ring. 

The  permeability  /*  of  iron  varies  from  500  to  1,000  or  more; 
and,  therefore,  the  effect*  of  placing  an  iron  core  in  a  coil  is  to  in- 
crease greatly  the  inductance  of  the  coil. 

The  iron  core  of  an  inductance  coil  to  be  used  with  alternating 
currents  should  be  laminated  to  reduce  eddy  currents  and  the  con- 


*The  permeability  fj,  of  a  given  sample  of  iron  is  not  constant,  but  decreases  in 
value  as  the  magnetizing  force  increases.  Therefore,  the  inductance  L  of  a  coil  having  an 
iron  core  is  not  a  definite  constant  quantity  as  is  the  inductance  of  a  coil  without  an  iron 
core. 


30  ALTERNATING-CURRENT  MACHINERY 

sequent  loss  of  energy,  and  to  prevent  excessive  heating  of  the  core. 

For  example,  the  inductance  of  the  field  coil  of  a  certain  shunt- 
wound  dynamo  is  7.5  henrys.  The  inductance  of  a  pair  of  No.  0, 
B.  &  S.  copper  line  wires  carried  at  a  distance  of  18  inches  apart 
on  a  pole  line  is  0.0035  henry  per  mile.  The  inductance  of  the 
secondary  coil  of  a  large  induction  coil  (X-ray  coil)  having  200,000 
turns  of  wire,  is  2,000  henrys. 

Series  and  Parallel.  The  inductance  of  two  or  more  coils  in 
series  is  equal  to  the  sum  of  the  individual  inductances. 

The  equivalent  inductance  of  two  or  more  similar  coils  in 

parallel,  such  as  the  similar  coils  on  an  armature,  is  equal  to— 

ft 

of  the  inductance  of  one  coil,  n  being  the  number  of  coils  connected 
in  parallel. 

Capacity.  It  has  been  pointed  out,  page  11,  that  a  charged 
condenser  has  an  elastic-like  reaction  analogous  to  the  elastic  reaction 
of  a  distorted  diaphragm.  The  elasticity  of  a  diaphragm  might 
be  measured  by  the  pressure  required  to  distort  it  to  the  extent  of 
producing  one  unit  of  increase  of  volume  in  the  space  on  one  side 
of  the  diaphragm  and  one  unit  of  decrease  of  volume  in  the  space 
on  the  other  side  of  the  diaphragm.  Similarly,  the  capacity  of  a 
condenser  may  be,  and  in  fact  is,  measured  by  the  electromotive 
force  required  to  force  one  unit  of  charge  of  electricity  into  one 
plate  of  the  condenser,  and  at  the  same  time  to  withdraw  one  unit 
of  charge  from  the  other  plate.  A  condenser  is  said  to  have  a  capacity 
of  1  farad*  when  one  volt  of  electromotive  force  pushes  one  coulomb 
of  electric  charge  into  the  condenser. 

The  charge  Q  pushed  into  a  condenser  by  a  steady  electro- 
motive force  E  is 

Q=CE  (18) 

in  which  C  is  the  capacity  of  the  condenser  in  farads,  Q  is  the  charge 
in  coulombs,  and  E  is  the  electromotive  force  in  volts.  The  electro- 
motive force  required  to  hold  a  given  charge  Q  in  the  condenser  is 

of  course  equal  to  -^r" 

Condensers,  to  have  a  large  capacity  (as  much  as  a  microfarad), 
are  usually  made  up  of  alternate  sheets  of  tinfoil  and  waxed  paper, 

*The  farad  is  an  exceedingly  'large  capacity,  and  capacities  encountered  in  practice 
are  usually  expressed  in  millionths  of  a  farad,  that  is,  in  microfarads. 


ALTERNATING-CURRENT  MACHINERY 


31 


TABLE  II 

Inductivitics  of  Dielectrics 

Referred  to  Air  as  Unity 


Glass 

3.00  to  10.00 

Mica 

4.00  to  8.00 

Vulcanite 

2.50 

Shellac 

2.93  to  3.3 

Paraffin 

1.68  to    2.30 

Turpentine 

2.15  to  2.43 

Beeswax 

1.86 

Petroleum 

2.02  to  2.19 

Gutta-percha 

3.3    to  4.9 

Rubber  (pure  India) 

2.12  to  2.24 

or  mica,  as  indicated  in  Fig.  41.  Alternate  metal  sheets  are  connected 
together  giving  two  terminals  as  shown,  thus  practically  forming 
two  metallic  plates  of  large  area.  A  condenser  having  a  capacity 
of  1  microfarad  contains  about  3,600  square  inches  of  tinfoil. 

Inductivity  of  Dielectric.  The 
capacity  of  a  condenser  of  given  dimen- 
sions depends  upon  the  material  used 
between  the  plates,  called  the  dielectric. 
The  quotient,  capacity  of  a  condenser 
with  given  dielectric  -r-  its  capacity 

with  air  as  the  dielectric,  is  called  the  inductivity  or  the  specific  induc- 
tive capacity  of  the  given  dielectric.  The  values  of  the  inductivity 
for  the  most  commonly  used  dielectrics  are  given  in  Table  II. 

Capacity  of  Condensers.    The  capacity  of  a  condenser  is  given 
by  the  equation 

=  885  k* 


Fig.  41.     Diagram  of  Condenser 


in  which  C  is  the  capacity  in  microfarads,  k  is  the  inductivity  of  the 
dielectric  used,  d  is  the  distance  in  centimeters  between  plates,  i.  e.y 
the  thickness  of  the  dielectric,  and  A  is  the  area  in  square  centi- 
meters of  both  sides  of  all  the  inner  plates  plus  the  area  of  the  inner 
surfaces  of  the  two  outside  plates. 

NOTE.  If  the  total  number  of  plates  =n,  A  will  be  the  area  of  both 
sides  of  (n-1)  plates;  or,  in  other  words,  it  is  necessary  to  provide  one  more 
plate  than  is  necessary,  using  both  sides,  to  equal  an  area  of  A  square  cen- 
timeters. 

Example.  It  is  required  to  design  a  plate  condenser  to  have  a  capacity 
of  1.5  microfarads  using  a  dielectric  of  oiled  paper  0.0043  inch  thick  and  tin- 
foil 0.0007  inch  thick. 


32  ALTERNATING-CURRENT  MACHINERY 

Assuming  for  the  oiled  paper  an  inductivity  of  2.67,  equation  (19)  gives 
for  the  total  active  plate  area 

1.5X1010X0.0043X2.54 

A  =  -  —  —  —  —  -  =  69400  square  centimeters  =  10759  square  inches 
885  X2.67 

If  65  plates  be  used,  the  area  of  one  side  of  each  plate  will  be 

10759 
^-^=84  square  inches 

BO  that  the  dimensions  of  one  active  plate  may  be  10|"X8"  and  the  area  of 
tinfoil  needed  will  be  84  X  65  =  5460  square  inches. 

The  capacity  of  an  ordinary  2-quart  Ley  den  jar  is  about  0.005 
microfarad.  The  capacity  of  an  average  submarine  telegraph  cable 
is  about  0.4  microfarad  per  nautical  mile.  The  capacity  of  a  pair  of 
transmission  lines  of  No.  0,  B.  &  S.  wires  placed  18  inches  apart  be- 
tween centers  on  poles,  is  0.036  microfarad  per  mile. 

Series  and  Parallel.  The  capacity  of  a  number  of  condensers 
in  parallel  is  equal  to  the  sum  of  the  individual  capacities. 

The  capacity  of  a  number  of  condensers  in  series  is 

C-——  -  (20) 


in  which  Cv  Cy  Cy  etc.,  are  the  individual  capacities,  and  C  is  the 

joint  capacity. 

Fundamental  Equations  of  the  A.  C.  Circuit.    An  alternator  Af 

Fig.  42,  delivers  an  alternating  current  of  /  amperes,  effective,  to 
a  circuit  consisting  of  a  resistance  of  R 
ohms,  an  inductance  of  L  henry  s,  and  a 
condenser  of  which  the  capacity  is  C  farads, 
all  connected  in  series.  It  is  to  be  remem- 
bered that  any  coil  having  inductance  has 
resistance  also;  that  is,  inductance  and  resist- 
ance are  practically  inseparable.  Neverthe- 

Fig.  42.    Alternating  circuit     less  inductance  and  resistance  are  essentially 

Containing  Resistance,  In-  . 

ductance,  and  Capacity   J     different  in  nature  and  in  their  effects,  and 
they  are  always  considered  separately,  so 

that  it  is  helpful  to  think  of  them  as  actually  separated  in  a  circuit, 
as  indicated  in  Fig.  42.  A  resistance  is  conventionally  represented 
thus,  -AA/W-  ;  an  inductance  thus,  —  'CJOQOCOO^-  ;  and  a  conden- 
ser thus, 


or — RI 


sfe 


ALTERNATING-CURRENT  MACHINERY  33 

The  current  in  the  circuit,  Fig.  42,  is  assumed  to  be  harmonic, 
that  is,  to  be  a  sine-wave  current,  and  this  current  is  represented  by 
the  line  01  in  Fig.  43. 

A  portion  of  the  electromotive  force 
of  the  alternator  is  used  to  overcome  the 
resistance  of  the  circuit.  The  portion  of 
the  electromotive  force  so  used  is  an  alter- 
nating electromotive  force  of  which  the 
effective  value  is  RI;  it  is  in  phase  with 
the  current,  and  is  represented  by  the  line 
RI  in  Fig.  43. 

A  portion  of  the  electromotive  force  of 
the  alternator  is  used  to  overcome  the  in-  Fig.  43.  vector  Diagram  of  Con- 

.     ,  „  1 ,         .         .  t  .  .  ditions  in  Fig.  42 

ertia  or  inductance  or  the  circuit  in  causing 

the  current  to  increase  and  decrease.  The  portion  of  the  electromotive 
force  so  used  is  an  alternating  electromotive  force  of  which  the 
effective  value  is  cuLI;  it  is  90  degrees  ahead  of  I  in  phase,  and  is 
represented  by  the  line  (oLI  in  Fig.  43.  The  quantity  a>  is  equal 
to  2/r  times  the  frequency  of  the  current  /. 

A  portion  of  the  electromotive  force  of  the  alternator  is  used 
to  overcome  what  we  have  previously  called  the  electro-elasticity  of 
the  condenser,  or,  in  other  words,  to  hold  electric  charge  on  the 
condenser  plates  at  each  instant.  The  portion  of  the  electromotive 
force  so  used  is  an  alternating  electromotive  force  of  which  the 

effective  value  is  — ;  it  is  90  degrees  behind  I  in  phase,  and  is  rep- 
resented by  the  line  —  in  Fig.  43. 

OJ\j 

The  total  electromotive  force  E  of  the  alternator  is  equal  to 
the  geometric  (or  vector)  sum  of  the  parts  RI,  (oLI,  and  — .  This 

C0\^/ 

vector  sum  is  formed  by  subtracting  —  from  ojLI,  since  it  is  opposite 

&>G 

to  uLI  in  direction,  and  then  adding  RI  and  (coLI -)  geometric- 
al 

ally,  as  shown  in  Fig.  44,  in  which  the  line  Oa  represents  (tuLI — , 


34  ALTERNATING-CURRENT  MACHINERY 

and  the  line  E  represents  the  geometric  sum  of  Oa  and  RI. 
From  Fig.  44  we  have,  by  geometry: 


or 


or 

I  =  E        (21) 


(The  quantity  a)L  --  ^  is  called  the  reactance  of  the  circuit.    The 
&>C 

term  a>L  is  often  called  inductance  reactance;  and  the  term  —  -  is 

cuC 

often   called   capacity    reactance.      Inductance  reactance  is  always 
positive,  and  capacity  reactance  is  always  negative.    It  is  convenient 

to  represent  the  reactance  a>L  --  —  of  a  circuit  by  the  single  letter 

o)C 

X;  that  is 

X=«L-1-C  (22) 

Therefore,  writing  X  for  coL  --  -in  equation  21,  we  have 

cuC 

I  =         E  (23) 

VR*  +  X2 

Furthermore,  from  the  right  triangle  in  Fig.  44  we  have 


or 

tan  8  =4  (24) 

K 

in  which  0  is  the  angle  of  phase  lag  of  the  current  I  behind  the 
electromotive  force  E;  X  is  the  reactance  of  the  circuit;  and  R  is 
the  resistance  of  the  circuit. 


ALTERNATING-CURRENT  MACHINERY 


35 


RI 


coC 


Resistance,  Reactance,  and  Impedance.  Consider  a  harmonic 
alternating  electromotive  force  E  which  produces  a  harmonic  alter- 
nating current  7  in  a  circuit. 
This  electromotive  force  may 
be  resolved  into  two  compo- 
nents, one  parallel  and  the 
other  perpendicular  to  I,  as  * 
shown,  for  example,  in  Fig.  ^° 
43.  The  component  of  E 
parallel  to  7  is  equal  to  RL 

The  resistance  of  an  al- 
ternating-current    Circuit     is  Fig   44     complete  Diagram  of  Conditions 

sometimes    defined    as    that 

factor  which,  multiplied  by  the  current,  gives  the  component  (of  the  elec- 
tromotive force)  which  is  parallel  to  I. 

The  component  of  E  perpendicular  to  7  is  equal  to  cuLI -> 

or  is  equal  to  XL 

The  reactance  of  an  alternating-current  circuit  may  be  defined 
as  that  factor  which,  multiplied  by  the  current,  gives  the  component 
(of  the  electromotive  force)  which  is  perpendicular  to  7. 

The  factor  i/7?-2  +  A'2,  which,  when  multiplied  by  the  current  7, 
gives  the  total  value  of  the  electromotive  force  E,  is  called  the 
impedance  (denoted  by  Z)  of  the  alternating-current  circuit.  Of 
course,  E  divided  by  Z  gives  the  value  of  the  current  7. 

NOTE.  Resistance,  reactance,  and  impedance  are  all  expressed  in  ohms; 
we  may,  for  example,  speak  of  10  ohms  of  resistance,  10  ohms  of  reactance, 
or  10  ohms  of  impedance.  Thus,  ohms  are  used  in  alternating-current  work 
to  express  the  three  essentially  different  things — resistance,  reactance,  and 
impedance;  and  a  specification  of  a  certain  number  of  ohms  is  not  intelligible 
unless  it  is  stated  whether  it  is  ohms  of  resistance,  ohms  of  reactance,  or  ohms 
of  impedance. 

The  reactance  and  the  impedance  of  a  circuit  depend  upon  the 
frequency  of  the  alternating  current,  as  well  as  upon  the  physical 
constants  L  and  C  of  the  circuit,  since  the  factor  a>  is  equal  to  2n 
times  the  frequency. 

The  reactance  of  a  circuit  may  be  positive  or  negative,  accord- 
ing as  coL  is  larger  than  or  less  than  — — •  When  reactance  is  posi- 


36 


ALTERNATING-CURRENT  MACHINERY 


tive,  the  inductance  reactance  coL  exceeds  the  capacity  reactance 

— ,  and  the  current  is  behind  the  electromotive  force  in  phase,  as 
ct)C 

shown  in  Fig.  44.  When  the  total  reactance  is  negative,  however, 
the  capacity  reactance  exceeds  the  inductance  reactance,  and  the 
current  is  ahead  of  the  electromotive  force  in  phase,  as  shown  in 
Fig.  45.  The  same  results  may  be  obtained  from  equation  (24).  If 
the  total  reactance  X  is  negative,  then  tan  6  is  negative,  which 
means  that  6  is  a  negative  angle,  or  that  the  electromotive  force  is 
behind  the  current  in  phase,  or  that  the  current  is  ahead  of  the  elec- 
tromotive force. 

Special  Cases  of  Electromotive  Force  and  Current  Relations.  A 
clear  understanding  of  the  following  examples  as  special  cases  of 
the  general  relations  of  electromotive  force  and  current  as  discussed 
on  pages  34  and  35,  depends  upon  the  following  facts: 

(a)  That  the  effect  of  inductance  in  an  alternating-current  circuit  be- 
comes negligible  when  the  inductance  is  very  small,  for  then  the  reactance 
<t)L  due  to  the  inductance  is  small,  and  the  portion  of  the  electromotive  force 
required  to  overcome  the  inductance,  namely,  toLI}  Fig.  43,  is  also  small. 


Fig.  45.     Conditions  of  Fig.  42  When  Total 
Reactance  is  Negative 

(b)  That  the  effect  of  a  condenser  in  an  alternating-current  circuit 
becomes  negligible  only  when  the  capacity  of  the  condenser  is  very  large,  for 

then  the  reactance due  to  the  condenser  is  small,  and  the  portion  of 

a)C 

the  electromotive  force  required  to  overcome  the  electro-elasticity  of  the  con- 
denser, namely,  — ,  Fig  43,  is  also  small. 
<i)C 

The  effect  of  an  inductance  may  be  rendered  negligible  by 
short-circuiting  it  with  a  low-resistance  wire;  and  the  effect  of  a 


ALTERNATING-CURRENT  MACHINERY 


37 


condenser  also  may  be  rendered  negligible  by  short-circuiting  it 
with  a  low-resistance  wire. 

CASE  A.  Non-inductive  or  non-reac- 
tive circuits.  A  circuit  which  does  not 
contain  a  condenser  and  does  not  have 
any  perceptible  inductance  is  called  a 
non-reactive  circuit.  The  term  non- 
inductive  is  frequently  used  in  the  sense 
in  which  non-reactive  is  here  defined. 
A  non-reactive  circuit  contains  only 
resistance;  and  the  total  electromotive 
force  required  to  produce  a  given  alter- 
nating current  7  in  a  non-reactive  circuit  of  which  the  resistance 
is  R  ohms,  is  RI  volts,*  and  the  electromotive  force  and  current 
are  in  phase  with  each  other.  Therefore,  the  relation  between  alter- 
nating electromotive  force  and  current  in  a  non-reactive  circuit  is 
precisely  the  same  as  in  the  case  of  direct  currents.  That  is 

E  =  RI 


Fig.  46.     Diagram  of  a  Non-React- 
ive Alternating  Circuit 


or 


7  = 


E 


(25) 


Fig.  46  represents  a  non-reactive  circuit  connected  to  an  alter- 
nator A;  and  Fig.  47  shows  the  relation  between  the  electromotive 
force  and  current. 

Any  circuit  in  which  the  outgoing  and  returning  wires  are 
very  near  together,  has  very  small  inductance.  An  ordinary  incan- 
descent lamp,  for  example,  has  a  negligible  inductance.  An  incan- 
descent lamp  the  resistance  of  Q  

which  when  hot  is  220  ohms,  I  E(=RIJ 

takes  half   an   ampere,   effective,       Fig.  47.     E.  M.  P.  and  Current  Relations  for 
,  ,  .a    Non- Reactive  Alternating  Circuit 

when  connected  to  alternating- 
current  supply  mains  between  which  the  effective  electromotive 
force,  is  110  volts;  the  current  is  in  phase  with  the  electromotive 
force  and  the  power  in  watts  is  equal  to  the  product  of  effective  volts 
times  effective  amperes,  or  55  watts.  Alternating-current  voltmeters 
are  always  made  as  nearly  as  possible  non-inductive. 


*Effective   values   are   always    understood   except   where   it  is  distinctly  stated  to  the 
contrary. 


38 


ALTERNATING-CURRENT  MACHINERY 


CASE  B.  Circuits  containing  resistance  and  inductance.  In  this 
case  the  reactance  X  (=coL)  is  positive,  and  the  current  lags  behind 

the  electromotive  force  in  phase,  as  before 

pointed  out.    The  tangent  of  the  angle 
j^ 

of  lag  is  equal  to  — ,  according  to  equa- 
H 

tion  (24);  therefore,  the  angle  of  lag  of 
the  current  is  small  when  X  is  small 
compared  with  R,  and  the  angle  of  lag 
approaches  90°  when  X  is  very  large 
compared  with  R. 

FiS-  48  ^presents  a  circuit  containing 
resistance  and  inductance,  connected  to 

an  alternator  A;  and  Fig.  49  shows  the  relation  between  the  electro- 
motive force  and  current. 

Examples.  1.  A  non-inductive  resistance  takes  10  amperes  from  220 
volt,  60-cycle  mains.  What  current  will  it  take:  (a)  from  220- volt,  25-cycle 
mains;  (b)  from  110-volt,  60-cycle  mains? 

Since  the  resistance  is  non-inductive,  the  impedance  is  equal  to  the 

resistance,  and  7  =  — — .    Solving  for  R  we  obtain  R  —  —  -  =22  ohms. 
R  10 

220 

(a)  The  current  will  be  I  =  —  =10  amperes. 

22 

(b)  Since  there  is  no  inductance  or  capacity  in  the  circuit,  the  im- 
pedance is  independent  of  the  frequency.     Therefore,  I—  —  =5  amperes. 

2.  An  impedance  coil  of  negligible  resistance  takes  3  amperes  from 
220-volt,  60-cycle  mains.  What  current  will  it  take  (a)  from  220-volt,  25- 
cycle  mains?  (b)  from  110-volt,  60-cycle  mains? 

In  this  case  the  resistance  R  is  zero,  so  that  the  impedance  is  equal  to 

El  OOft 

the  reactance,  and  7  =  — .  Solving  for  X  we  obtain  X  =  —(  -   =  73.3  ohms 
X  3 

when  the  frequency  is  60  cycles.  , 

(a)  When  the  frequency  is  reduced  from  60  to  25  cycles,  the  reactance 
X  is  reduced  in  the  same  ratio  since  X  =2nfL.  Therefore,  the  reactance  at 

25  "220  X  60 

25  cycles  is  73.3  X  —  ohms.    The  current  is  7  = —  =  7.2  amperes. 

60  73.3  X  25 


(b)     Since  frequency  is  again  60  cycles,  X  =  73.3,  and  7  =• 


110 
73.3 


=  1.5 


amperes. 

A  coil  of  wire  usually  has  a  very  considerable  inductance,  es- 
pecially if  it  is  wound  on  a  laminated  iron  core.     In  fact,  a  coil 


ALTERNATING-CURRENT  MACHINERY 


39 


wound  on  a  laminated  iron  core  usually  has  so  large  a  reactance 
X  (=^L),  that  the  angle  0,  Fig.  49,  is  very  nearly  90°. 

Example.  A  certain  coil  has  a  resistance  of  2  ohms  and  an  inductance 
of  0.3  henry  when  provided  with  a  laminated-iron  core.  This  coil  is  connected 
to  an  alternator  giving  1,000  volts  effective  electromotive  force  at  a  frequency 
of  133  cycles  per  second,  so  that  the  factor  o>  is  equal  to  2n  X  133,  or  835.7 
radians  per  second;  the  reactance  of  the  coil  is  835. 7 X  0.3,  or  250.7  ohms;  the 

impedance  is  V22+  250.72  or  250.7  ohms;  the  current  is  — ,  or 3. 989am- 

250.7 

peres;  the  current  lags  about  89|°  behind  the  electromotive  force;  and  the 
power  delivered  to  the  coil  is  1,000  volts  X  3.989  amperes  X  cos  89|°  (0.008), 
which  is  equal  to  31.9  watts  (  =  I2R).  The  product  El,  sometimes  called  ap- 
parent watts,  is  equal  to  3,989  volt  amperes. 

This  example  illustrates  one  remarkable  feature  of  alternating 
currents — namely,  the  very  small  amount  of  actual  power  that  is 
delivered  to  a  circuit  of  large  reactance  even  though  the  electro- 
motive force  is  large  and  the  current  considerable.  In  the  case  of 
a  direct  current,  3.989  amperes  taken  from  1,000-volt  mains  would 
mean  an  actual  delivery  of 
3,989  watts  of  power,  while 
in  the  above  case  the  actual 
power  delivered  is  only  31.9 
watts.  The  ratio  true  watts  -=- 
apparent  watts  is  called  the 
power  factor  of  a  circuit;  and 


U>LI 
or 
XI 


RI 


Fig.  49.     Diagram  of  E.  M.  F.s  and  Current 
Relations  for  Condition  in  Fig.  48 


in  case  of  the  coil  here  under 
discussion,  this  ratio  is  equal 
to  about  0.008  (=cos  0). 

One  never  encounters  in  practice  a  circuit  in  which  the  reactance 
is  so  large  compared  to  the  resistance  as  in  the  above  example;  that 
is,  one  never  encounters  one  in  which  the  power  factor  is  so  small 
as  0.008.  Cases  are  often  met  with,  however,  where  the  reactance 
is  from  two  to  ten  times  as  large  as  the  resistance.  Thus,  one  of  the 
primary  windings  of  a  certain  110-volt  induction  motor  has  a  resist- 
ance of  0.7  ohm  and  a  reactance  of  4.2  ohms.  With  zero  load  this 

110  110 

circuit,  equation  (23),  takes  — .  =  A  OKQ,  or  25.83  amperes; 


4.258 : 


the  angle  of  phase  lag  of  the  current,  according  to  equation  (24),  is 
about  80J°;  and  the  power  factor  of  the  circuit  is  0.164. 


40 


ALTERNATING-CURRENT  MACHINERY 


A  circuit  which  contains  a  coil  wound  on  an  iron  core  takes 
more  power  than  is  expended  in  the  mere  heating  of  the  wire,  namely, 

I2R,  for  some  power  is  con- 
sumed in  the  iron  core  on 
account  of  magnetic  hyster- 
esis and  eddy  currents.  In 
the  above  examples  this  con- 
sumption of  power  in  an 
iron  core  is  neglected  for 
the  sake  of  simplicity. 
The  term  equivalent  resist- 

Fig.  50..  Circuit  Containing  Resistance  and  Capacity       aU€f  /S  USed  *°  Designate   a 

fictitious   resistance   which 

when  multiplied  by  72  gives  the  actual  power  consumed  by  such  a 
circuit,  including  both  the  power  consumed  in  heating  the  wire,  and 
that  consumed  by  core  loss.  The  equivalent  resistance  of  such  circuits 
has  a  value  larger  than  the  mere  resistance  of  the  copper  winding. 
CASE  C.  Circuits  containing  resistance  and  a  condenser.  In 

this  case  the  reactance  ^T  (= -)   is  negative,   and  the  current 

leads  the  electromotive  force  in  phase,  as  before  pointed  out.    The 

V 

tangent  of  the  angle  of  lead  is  equal  to  —  according  to  equation  (24). 

H 

Therefore,  the  angle  of  lead  of  the  current  is  small  when  X  is  small 

compared  with  R,  that  is, 
when  a>C  is  large;  while  the 
angle  of  lead  of  the  current 
approaches  90°  when  X  is 
large  compared  with  R,  that 
is,  when  coC  is  small. 

Fig.  50  represents  a  circuit 
containing  resistance  and  a 
condenser  connected  to  an 
alternator  A;  and  Fig.  51  shows  the  relation  between  the  electromo- 
tive force  and  current. 

Example.  A  condenser  with  a  capacity  of  2  microfarads  (which  is 
large,  as  condensers  go)  is  connected  to  alternating-current  mains  through  a 
resistance  coil  of  200  ohms.  The  effective  electromotive  force  between  the 


Ri 


XI 
or 

coC 


Fig.  51.     Diagram  of  E.  M.  F.  and  Current 
Relations  for  Conditions  of  Fig.  50 


ALTERNATING-CURRENT  MACHINERY  41 


mains  is  1,000  volts,  and  the  frequency  is  133  cycles  per  second,  so  that  the 
factor  CD  is  equal  to  2nX  133  or  835.7  radians  per  second;  the  reactance  of  the 

106 

condenser  is =  598.3  ohms  (negative) ;  the  impedance  of  the  circuit  is 

2  X  835.7 

/^_^_o    '  "^     ~    "  3 

^200  -f  598.3  =630.85  ohms;  the  current,  according  to  equation  (23),  is 
1.585  amperes;  the  current,  according  to  equation  (24),  is  71°  31'  ahead  of  the 
electromotive  force  in  phase;  and  the  power  delivered  to  the  circuit  is  1,000 
volts  X  1.585  amperesXcos  71°  31',  which  is  equal  to  502.5  watts  (=/2#). 

If  the  above  condenser  is  connected  to  the  1,000-volt,  133-cycle  mains 

through  a  wire  of  negligible  resistance,  then  the  current  will  be  -  — • 

598.3  ohms 

or  1.671  amperes;  the  current  will  be  very  nearly  90°  ahead  of  the  electromo- 
tive force  in  phase;  the  power  factor  cos  d,  will  be  nearly  zero;  and  of  course 
the  power  delivered  to  the  condenser  will  be  nearly  zero. 

A  circuit  containing  a  condenser  takes  a  little  more  power 
than  is  expended  in  the  mere  heating  of  the  wire,  namely,  I2R,  for 
some  power  is  consumed  in  the  insulating  material  between  the 
condenser  plates.  This  power  consumed  in  the  dielectric  is  said  to 
be  due  to  dielectric  hysteresis.  In  the  above  examples,  this  consump- 
tion of  power  in  the  insulating  material  of  a  condenser  is  neglected 
for  the  sake  of  simplicity. 

CASE  D.    Circuit  in  which  the  inductance  reactance  a>L  is  balanced 

by  the  capacity  reactance  — .    In  this  case  equation  (21)  reduces  to  /= 

E' 

— ;  that  is,  the  electromotive  force  acting  upon  the  circuit  has  to 
R 

overcome  resistance  only,  as  in  the  case  of  the  non-reactive  circuit. 
This  case  in  which  a>L is  equal  to  zero  is  considered  again  in 

(JL)\J 

the  following  article  on  resonance. 

Electrical  Resonance.  Consider  a  circuit,  like  the  one  shown 
in  Fig.  42,  containing  a  given  resistance  R,  a  given  induction  L, 
and  a  given  capacity  C.  Suppose  that  the  alternator  A  is  at  first 
run  at  very  slow  speed  so  as  to  give  very  low  frequency,  and  is  then 
gradually  increased  in  speed  so  as  to  cause  the  frequency  to  increase. 
This  gradual  increase  of  frequency  will  cause  a  gradual  increase  in 
the  value  of  the  factor  a>  (equal  to  2n  times  the  frequency) ;  and  as 
w  increases,  the  following  relations  between  inductance  reactance 

coL  and  capacity  reactance  — - -  will  obtain: 


42  ALTERNATING-CURRENT  MACHINERY 

(a)     At  first,   when  the  frequency  is  very  low  (few  cycles  per 
second),  the  value  of  to  is  small.    Therefore,  the  inductance  reactance 

toL  is  small;  the  capacity  reactance  —  is  large;  and  the  total  net 

toC 

reactance  toL is   negative,   and  very  nearly  the  same  as  — - 

,  toC  toC 

alone. 

(6)     As  the  frequency  increases,  the  value  of  to  increases.    There- 
fore, the  inductance  reactance  toL  increases;  the  capacity  reactance 

— -  decreases;   and   the  total   net  reactance  toL increases   in 

toC  toC 

value  on  account  of  the  increase  of  toL,  and  also  on  account  of  the 

decrease  of  — .    For  a  certain  critical  value  of  the  frequency,  toL 
toL> 

becomes  equal  to  — ,  so  that  the  total  net  reactance  is  then  zero. 
That  is 

wL =0 


or 


or 


or 


VLC 

or,  since  to  equals  2-nf',  we  have 


/'  =  ~  (26) 


in  which/'  is  the  critical  value  of  the  frequency  for  which  inductance 
reactance  is  balanced  by  capacity  in  the  given  circuit,  L  being  the 
inductance  of  the  circuit  and  C  the  capacity  of  the  condenser.  See 
Fig.  42. 


ALTERNATING-CURRENT  MACHINERY 


43 


MU 

100 

20 
n 

V 

-_ 

Fig.  52.     Graphical  Relation  of  Current  and  Fre- 
quency; E.  M.  F.  Constant 


(c)  As  the  frequency  increases  beyond  the  critical  value  /',  the 
inductance  reactance  coL  continues  to  increase;  the  capacity  reactance 

continues  to  decrease;  and  the  net  reactance  ajL ~,  now  positive 

in  value,  continues  to  increase  in  value. 

Now,  imagine  the  electromotive  force  of  the  alternator  to  be 
constant  in  value,  although  increasing  in  frequency  as  the  alter- 
nator is  speeded  up.*    The  current  in  the  circuit  will  at  first  increase 
with  increasing  frequency  until 
the  critical  frequency  /',  equa- 
tion (26),  is  reached,  and  then 
the   current  will  decrease  in 
value    as    the   frequency    in- 
creases, a  maximum  value  of 
current  being  produced  at  the 
critical    frequency   /'.      This 
production  of  a  maximum  cur- 
rent at  the  critical  frequency 
/'  is  called  electrical  resonance.  At  critical'frequency  the  reactance  ajL— 

1  •  J? 

—^  is  zero;  and  the  general  equation  (21)  reduces  to  I  =  — , 
toC  ft 

as  explained  in  CaseD,  page  41.  That  is,  the  value  of  the  current  at 
the  critical  frequency  is  determined  solely  by  the  resistance  of  the  circuit 
and  for  a  given  resistance  has  its  maximum  value. 

The  variation  of  current  in  a  circuit  like  that  shown  in  Fig.  42, 
with  increasing  frequency,  electromotive  force  being  kept  constant 
in  value,  is  shown  graphically  in  Fig.  52,  which  is  calculated  from 
the  following  data:  E=200  volts  (effective);  R=2  ohms;  L=  0.352 
henry;  and  £=20  microfarads. 

The  critical  frequency  corresponding  to  these  values  of  L  and 
C  is  60  cycles  per  second,  according  to  equation  (26).  The  maxi- 
mum point  of  the  curve  is  not  a  cusp,  as  would  appear  from  the 
figure;  but  the  curve  is  rounded  at  the  top,  the  figure  being  drawn 
on  too  small  a  scale  to  show  it. 

It  should  be  remarked  that  the  conditions  for  complet  reeson- 
ance  can  be  obtained  only  when  C  and  L  are  constant  and  con- 


*In  practice  this  condition  could  be  realized  by  adjusting  the  field  rheostat  of  the 
alternator,  or  of  the  exciter,  so  as  to  reduce  the  exciting  current  as  the  speed  of  the  alter- 
nator  is  increased. 


44  ALTERNATING-CURRENT  MACHINERY 

centrated  (not  distributed),  and  when  the  electromotive  force  is 
harmonic.  If  the  electromotive  force  is  non-harmonic,  it  means 
that  it  is  composed  of  a  number  of  sine  waves  having  different  fre- 
quencies. It  is  evident,  therefore,  that  ioL  can  never  be  made 

exactly  equal  to  the  fraction  —=  unless  there  is  but  one  fundamental 

CO\j 
(i) 

frequency  equal  to  /=  —  . 

2n 

Multiplication  of  E.  M.  F.  by  Resonance.  When  resonance 
exists  in  a  circuit  containing  an  inductance  and  a  condenser  in  series, 
the  alternating  electromotive  force  coLI  between  the  terminals  of 

the  inductance,  and  the  alternating  electromotive  force  — -  between 

wC 

the  terminals  of  the  condenser,  may  each  be  much  greater  than  the 
alternating  electromotive  force  RI  which  acts  upon  the  circuit.  This 

fact  is  easily  understood  by  means  of  the 
mechanical  analogue.      If  even  a  very 

/  weak  periodic  force  act  upon  a  weight 

which  is  suspended  from  a  spiral  spring, 
the  weight  will  be  set  into  violent  vibra- 

?/=£"  / 

tion,  provided  the  frequency  of  the  force 
is  the  same  as  the  proper  frequency  of 
oscillation  of  the  body.  The  forces  acting 
on  the  spring  may  reach  enormously 
greater  values  than  the  periodic  force 
Fig  53.  Diagram  showing  Muitipii-  which  maintains  the  motion  of  the  sys- 

cation  of  E.  M.  F.s  by  Resonance  * 

tern.    Moreover,  the  forces  which  act  up- 

ori  the  weight  to  produce  its  up-and-down  acceleration  may  reach  values 
very  much  larger  than  the  periodic  force  which  maintains  the  motion. 

Example.  A  coil  having  an  inductance  of  0.352  henry  and  a  resistance 
of  2  ohms,  and  a  condenser  of  20  microfarads  capacity,  are  connected  in  series 
between  alternating-current  mains.  The  critical  frequency  of  this  circuit  is 
60  cycles  per  second,  according  to  equation  (26).  The  electromotive  force 
between  the  mains  is  200  volts,  and  its  frequency  is  60  cycles  per  second. 

The  current  in  the  circuit  is  —          — .  or  100  amperes,  according  to  equation 

2  ohms 

(21);  the  effective  electromotive  force  between  the  condenser  terminals  is 
13,270  volts  effective  (  =  —~):  and  the  electromotive  force  between  the 

ajC 

nals  of  the  inductance  is  also  13,270  volts  effective  (  =  ^L7). 


ALTERNATING-CURRENT  MACHINERY 


45 


The  multiplication  of  electromotive  force  by  resonance  may  be 
clearly  understood  with  the  help  of  the  clock  diagram,  Fig.  53. 


a 

a 

a 

b 

b 

b 

Fig.  54.     An  Alternator  Delivering  Current  to  a  Long  Transmission  Line 

The  electromotive  force  toLI  required  to  overcome  inductance 
reactance  is  equal  and  opposite  to  the  electromotive  force  —  re- 
quired to  overcome  capacity  reactance,  as  shown  in  Fig.  53,  so 
that  the  geometric  sum  of  coLI,  — ,  and  #7  is,  simply,  RI.  A 

transmission  line  has  both  inductance  and  capacity  and,  there- 
fore, electrical  resonance  may  occur  on  a  transmission  line.  The 
phenomena  of  a  transmission  line,  however,  are  very  greatly  com- 
plicated by  the  fact  that  the  capacity  is  distributed;  and  a  simple 
explanation  of  line  reasonance  can  be  given  only  by  approxi- 
mation. 

For  example,  an  alternator  A,  Fig.  54,  delivers  current  to  a 
long  transmission  line,    The  resonance  effects  are  nearly  independent 


000000000000 


Fig.  55.     Diagram  of  Conditions  Shown  in  Fig.  54  with  Transmission 
Lines  Insulated  from  Each    Other 


of  whether  the  receiving  apparatus  at  the  end  of  the  line,  viz,  at 
B,  is  connected  into  the  circuit  or  not.  We  shall,  therefore,  consider 
that  the  receiving  apparatus  is  disconnected  and,  furthermore, 
that  the  ends  of  the  two  transmission  lines  are  insulated  from 
each  other. 


46 


ALTERNATING-CURRENT  MACHINERY 


Fig.  56.    Circuit  Showing  Conden- 
ser and  Inductance  in  Parallel 


A  first  approximation  to  the  behavior  of  the  line  may  be  ob- 
tained by  looking  upon  the  distant  end  of  the  line  b  b  b  as  a  con- 
denser purely  and  simply,  while  the  near  end  of  the  line  aaa  is 

looked   upon  simply  as  an  inductance. 

The  transmission  line  shown  in  Fig.  54 
is  then  equivalent  to  the  combination 
shown  in  Fig.  55,  which  is  identical  with 
the  combination  shown  in  Fig.  42.  The 
value  of  L  may  be  taken  as  the  induc- 
tance of,  say,  half  the  length  of  the 
line;  and  the  capacity  C  may  be  taken 
as  the  capacity  of  the  distant  half  of 
the  transmission  line.  Then,  if  the  alternator  A  gives  a  frequency 
equal  to  the  critical  value  of  these  values  of  L  and  C,  as  per  equa- 
tion (26),  we  shall  have  resonance,  and  the  electromotive  force  be- 
tween the  lines  at  the  distant  end  (between  the  terminals  of  (7, 
Fig.  55)  may  be  greatly  in  excess  of  the  electromotive  force  of  the 
alternator  A.  This  condition  actually  occurs  in  the  practical  opera- 
tion of  long  transmission  lines;  and  it  is  not  an  uncommon  thing 
to  have  as  much  as  11,000  volts  at  the  receiving  end  of  a  long 
transmission  line  when  the  electromotive  force  of  the  generator 
is  but  10,000  volts. 

Multiplication  of  Current  by  Resonance.  An  alternator  A ,  Fig.  56, 
delivers  current  to  a  circuit  which  divides  at  the  points  a  and  6  into 

two  branches,  one  branch  containing  an 
inductance  L  and  the  other  branch  con- 
taining a  capacity  (7,  as  shown.  The 
two  branches  constitute  a  closed  circuit 
in  and  of  themselves;  and  if  the  fre- 
quency of  the  alternator  is  equal  to  the 
critical  frequency  of  the  circuit  constitu- 
ted by  the  two  branches,  that  is,  if  the 
frequency  of  the  alternator  is  equal  to 

=,  as  per  equation  (26),  then  the 


Fig.  57.   Vector  Diagram  for  Multipli- 
cation  of  Current  by  Resonance 


/  -  , 

small  current  I  from  the  alternator  will  divide  into  two  currents 
/i  and  72  in  the  respective  branches,  and  the  currents  71  and  72 
may  each  be  very  much  larger  in  value  than  the  undivided  cur- 


ALTERNATING-CURRENT  MACHINERY 


47 


main 


550  vo/ts 
133    cyc/es 

per  second 


main 


rent  7.  The  fact  is  that,  because  of  resonance,  a  very  large  current 
is  made  to  surge  back  and  forth  around  the  closed  circuit  formed 
by  the  two  branches. 

The  multiplication  of  cur- 
rent by  resonance  may  be  clearly 
understood  with  the  help  of  the 
clock  diagram,  Fig.  57.  The  line 
OE  represents  the  electromotive 
force  between  the  branch  points 
and  a  b,  Fig.  56 ;  the  line  7X  rep-  Fig.  58.  Diagram  of  circuit  showing  Muiti- 

.  .  plication  of  Current  by  Resonance 

resents  ttye  lagging  current  which 

the  electromotive  force  E  produces  in  the  branch  containing  the 
inductan  fjfc  the  line  72  represents  the  leading  current  which  the 
electromom^e  force  E  produces  in  the  branch  containing  the  con- 
denser; and  the  line  7,  which  is  the  geometric  sum  of  7X  and  72, 
represents  the  total  current  in  the  undivided  part  of  the  circuit  in 
Fig.  56. 

For  example,  three  similar  32-candle-power  incandescent  lamps 
A,  B,  and  7),  Fig.  58,  each  having  100  ohms  resistance,  are  con- 
nected as  shown,  to  550-volt  mains;  L  is  an  inductance  of  0.597 
henry;  and  C}  a  capacity  of  2.49  microfarads.  Then  the  current 
flowing  through  the  lamp  A  is  not  quite  0.4  ampere,  while  one 
ampere  of  current  flows  through  each  of  the  lamps  B  and  D. 

Miscellaneous  Considerations.  Condenser  as  Compensator  for 
Lagging  Current.  An  alternator  may  be  designed  to  develop  a  cer- 
tain effective  electromotive  force  E,  and  to  deliver  a  certain  effective 

current   7,  at   full   load.     The 

U 


permissible    power    output    of 

such    an   alternator   would    be 

El    watts     to    a    non-reactive 

circuit     having     unity    power 

factor   (cos   0=1);  but  if  the 

receiving  circuit  is  reactive,  the 

permissible  power  output  of  the 

alternator    is    only    El   cos   6, 

where  the  power  factor  (cos  6)  may  in  practice  have  a  value  of  .8 

or  less.     If  a  condenser  C  of  sufficiently  large  capacity,  Fig.  59,  is 

connected  across  the  terminals  of  an  alternator  A  in  parallel  with 


Fig.  59.     Diagram  of  Condenser  Compen- 
sating for  Lagging  Current 


48  ALTERNATING-CURRENT  MACHINERY 

an  inductive  receiving  circuit  RL,  the  effect  of  L  will  be  neutralized; 
the  current  delivered  by  A  will  be  in  phase  with  the  electromotive 
force  of  A;  and  the  permissible  power  output  will  be  EL  The 
condenser  is  said  to  compensate  for  the  lagging  current  taken  by 
the  inductive  receiving  circuit. 

The  compensation  produced  by  a  condenser  is  due  to  the  fact 
that  the  alternating  current  taken  by  it  is  ahead  of  the  electromotive 
force  in  phase,  while  that  taken  by  the  reactive  receiving  circuit  is 
behind  the  electromotive  force  in  phase. 

Another  advantage,  aside  from  the  increase  of  the  permissible 
power  output  of  the  generator,  that  would  result  from  «this  com- 
pensating of  lagging  current  by  means  of  a  condenser,  is  that  the 
electromotive  force  of  the  alternator  would  not  fall  JMSO  much 
with  increase  of  load  as  is  the  case  when  lagging  cil^rent  is  not 
compensated  for.  The  cost,  however,  of  large  condensers  is  so 

great  that  their  use  for  compensation  of 
lagging  current  is  not  commercially  prac- 
ticable, as  may  be  seen  from  the  follow- 
ing discussion: 

Let  Ir  be  the  current  delivered  to  the 
receiving  circuit  RL,  Fig.  59.    Let  I0  be 
the  current  delivered  to  the  condenser  C; 
this  current  is  90  degrees  ahead  of  E  in 
Fig-dLnIesto^\anerr1g.°5f9Con-   .phase.  Let  Ia  be  the   current  delivered 

by  the  alternator  A.      It  is  desired  that 

Ia  be  in  phase  with  E,  as  shown  in  Fig.  60.  Let  cos  0  be  the  power 
factor  of  the  receiving  circuit  RL. 

From  Fig.  60  it  is  evident  that  I0  is  equal  and  opposite  to  that 
component  of  Ir  which  is  at  right  angles  to  E,  namely,  Ir  sin  0. 
Therefore 

Ic  =  7P  sin  0 

A 

Now,   7.  is  equal  to  1   (=  uCE),  that  is,  is  equal  to  the  electro- 

wC 
motive  force  between  the  condenser  terminals  divided  by  the  reactance 

of  the  condenser.      The  value  of  sin  0  is    /—£-          J  and    Ir= 


ALTERNATING-CURRENT  MACHINERY  49 

E 


^;  so  that,  substituting  the  values  of  7P  and  sin  6,  the 


above  equation  for    I0  becomes 

coLE 


CO 


C  E  = 


R2  +  w2  L2 
whence 


C  = 


L 


R2  +w2  L2 


(27) 


in  which  R  is  the  resistance  (in  ohms)  of  the  receiving  circuit;  L  is 
the  inductance  (in  henry s)  of  the  receiving  circuit;  a;  is  a  factor 
equal  to  2rc  times  the  frequency  in  cycles  per  second;  and  C  is  the 
capacity  (in  farads)  of  the  condenser  required  to  compensate  for 
the  lagging  current  delivered  to  the  receiving  circuit.  Suppose 
an  alternator  having  an  electromotive  force  of  1,100  volts  and  a  fre- 
quency of  60  cycles  per  second,  delivers  102.4  amperes  of  current 
to  a  receiving  circuit  of  which  the  power  factor  is  0.871  (9.35  ohms 
resistance  and  0.014  henry  inductance).  The  capacity  of  a  con- 
denser, which  will  compensate  for  the  lagging  current  in  this  case, 
may  be  calculated  from  equation  (27)  as  follows : 

C=  0-014 

9.352 +(27rX60)2X0.0142 

=  0.0001214  farad  =  121.4  microfarads 

This  condenser  would  take  from  the  mains  a  current  of  50.34  am- 
peres, which  would  be  90  degrees  ahead  of  the  e.  m.  f.  in  phase, 
and  this  current  would  be  equal  and  opposite  to  the  wattless  com- 
ponent of  the  102.4  amperes  of  current  delivered  to  the  inductive 
circuit.  Such  a  condenser  would  require  about  114,000  leaves  of 
tinfoil  8  inchesXIO  inches,  separated  by  114,000  leaves  of  paraffined 
paper,  each  0.03  inch  in  thickness.  This  would  give  a  stack  of 
condenser  leaves  of  about  400  feet  total  thickness;  and  the  cost  of 
material  and  labor  would  be  at  least  $10  per  microfarad. 

Such  a  condenser  would  be  impracticable  but  a  synchronous 
motor  with  over-excited  field  magnets  behaves  like  a  large  con- 
denser in  that  it  takes  an  armature  current  which  is  ahead  of  the 
electromotive  force  in  phase.  The  synchronous  motor  is  often  used 
in  practice  to  compensate  for  lagging  line  current  and  when  so 
operated  is  called  a  rotary  condenser. 


50 


ALTERNATING-CURRENT  MACHINERY 


Circuits  in  Series.    An  alternator  A,  Fig.  61,  delivers  current 
to  two  coils  (or  elements)  in  series  as  shown.    Let  R^  be  the  resist- 

Let   RZ  be  the  resistance 


ance  and  X^  the  reactance  of  coil   1. 


ma/n 


Fig.  61.     Two  Elements  in  Series 
in  Alternating  Circuit 


E, 


Fig.  62.     Diagram  of  E.  M.  F. 
Conditions  for  Fig.  61 


and  X2  the  reactance  of  coil  2.  Let  7  be  the  current  flowing 
through  the  circuit;  E  the  electromotive  force  between  the  mains; 
EI  the  electromotive  force  between  the  terminals  of  coil  1;  and 
E2  the  electromotive  force  between  the  terminals  of  coil  2.  Let 
0  be  the  phase  difference  between  E  and  /;  dl  the  phase  difference 
between  Et  and  /;  and  02  the  phase  difference  between  E2  and  7, 
as  shown  in  Fig.  62. 

V 

Of  course  0\  is  the  angle  whose  tangent  is  —  ;  #2  is  the  angle 

HI 


•\7 

whose  tangent  is  —  2;  and  6  is  the  angle  whose  tangent  is 
Rz 

according  to  equation  (24). 


/  Y        [      Y  \ 


Example.  Two  impedance  coils  have  resistances  of  5  and  8  ohms  and 
inductances  of  0.01  and  0.2  henry,  respectively.  If  these  coils  are  connected 
in  series  across  220-  volt,  60-cycle  mains,  find:  (a)  the  current;  (b)  the  volt- 
ages impressed  across  the  coils;  and  (c)  the  phase  relations  between  the  cur- 
rent and  the  voltages  impressed  across  the  coils. 

SOLUTION.  We  have  JBi=5,  #2  =  8,  #  =  220,  /  =  60,  Li=0.01,  L2=0.2. 
Then  w=2^/=27rX  60  =  377  radians  per  second.  The  reactance  of  coil  1  is 
Xi  =  ojLi  =  377  X  0.01  =  3.77  ohms,  and  the  reactance  of  coil  2  is  X2  = 
ojLi  =  377  X  0.2  =  75.4  ohms.  The  total  impedance  of  the  circuit  is  Z== 

>/(#!  +  #2)2  +  (Xi+  X2)2=  V(5+  8)2  +  (3.774-  75.4)2  =  80.23  ohms,  so  that 
the  current  is 


220 
8O23 


2.74  amperes 


ALTERNATING-CURRENT  MACHINERY  51 

To  find  the  voltages  E i  and  E 2.     (See  Figs.  61  and  62.)     The  impedance 


of  coil  1  is  Z\  =  -v/52  +  3.772  =  6.262  ohms  and  the  impedance  of  coil  2  is  Z2  = 
-v/82  +  75. 42  =  75.82  ohms.     The  magnitude  of  the  voltage  Ei  is 

Ei=  7Zi=  2.74  X  6.262  =  17.16  volts, 
and  the  magnitude  of  the  voltage  E2  is 

E*  =  IZ2  =  2.74  X  75.82  =  207.7  volts 
The  phase  angle  0i  between  the  current  and  E\  is  obtained  from  the 

V  Q  77 

relation  tan  Oi=  —-  —  -    -  =  0.754,  whence 
Ri          5 

01  =  37°  1' 

Similarly  the  phase  angle   02  between  /  and  E2  is   found   to  be  02  =  tan"1 
=  tan-1  9.425,  or 

02  =83°  56' 
and  tan  0  =  — - — ^-8  =  — '—  =6.090,  whence 

-fil+    XV2 

0  =  80°  41' 

Referring  to  Fig.  62  it  is  evident  that  by  taking  the  line  (or  vector) 
representing  the  current  7  as  the  axis  of  reference,  the  lines  representing  the 
electromotive  forces,  E\,  E2,  and  E  may  each  be  resolved  into  two  components, 
one  parallel  to  or  in  phase  with  /  and  the  other  perpendicular  to  or  in  quad- 
rature with  /.  Then  the  sum  of  the  components  of  E\  and  E«  parallel  to  / 
will  be  equal  to  the  component  of  E  parallel  to  7,  and  similarly  the  sum  of 
the  components  of  Ei  and  E2  perpendicular  to  7,  will  be  equal  to  the  vertical 
component  of  E.  Expressing  these  statements  in  equation  form  and  substi- 
tuting the  values,  we  have  the  following: 

• 

(Ei  cos  0i)   +    (E*  cos  02)         (E  cos  0) 
(17.16  cos  37°  1')+  (207.7  cos  83°  56')  =  (220  cos  80°  40..3') 

(13.71)    +     (21.98)     =  (35.69  appro*. ) 
and 

(Ei  sin  0i)        +         (Ei  sin  02)        =          (E  sin  0) 
(17.16  sin  37°  1')  +(207.7  sin  83°  56')  =    (220  sin  80°  40.5') 
(10.34)       .     +  (206.7)  =        (217approx.) 

Circuits  in  Parallel.  An  alternator  A,  Fig.  63,  delivers  current 
to  two  coils  (or  elements)  in  parallel  as  shown. 

Let  RI  be  the  resistance,  and  Xi  the  reactance  of  coil  1; 
let  R2  be  the  resistance,  and  X2  the  reactance  of  coil  2;  let  E 
be  the  electromotive  force  between  the  coil  terminals;  let  /i  be 
the  current  in  coil  1,  72  the  current  in  coil  2,  and  /  the  total  current; 
and  let  6  be  the  angle  of  phase  difference  between  E  and  /;  0\  the 


52 


ALTERNATING-CURRENT  MACHINERY 


phase  angle  between  7X  and  E;  and  02  the  angle  between  72  and  E, 
as  shown  in  Fig.  64; 

The  angles  0i,  02,  are  known  from  the  relations 


tan  0!= 


X, 


E 


and/2= 


The  branch  currents  are:  7i= 

but  since  they  are  not  in  phase,  they  cannot  be  added  algebraically 
to  obtain  the  resultant  current  7,  but  must  be  added  geometric- 
ally as  is  done  in  Fig.  64.  The  magnitude  of  7  may  be  found  by 
trigonometry  thus: 


7=  V  I  \ 


+  l    +  27i  72  cos  (02- 


In  the  case  of  circuits  in  parallel  the  voltage  across  each  branch 
is  the  same,  and  it  is,  therefore,  convenient  to  determine  all  currents 
by  their  magnitudes  and  their  relations  with  respect  to  this  common 


Fig.  63.     Two  Elements  in  Parallel  in  Alternating 
Circuit 


voltage.  Thus  in  Fig.  64  draw  the  line  OE  to  represent  the  voltage 
in  magnitude;  it  is  convenient  to  draw  it  horizontal  since  the  current 
vectors  in  the  clock  diagram  are  to  be  referred  to  this  vector  voltage 
as  the  axis  of  reference.  The  vectors  Oh  and  072  are  then  drawn 
from  0  to  represent  the  currents  /i  and  72  in  magnitude  and  with 
the  proper  directions  as  determined  by  the  phase  angles  61  and  02. 

The  magnitude  and  phase  of  the  resultant  current  7  may  be 
obtained  graphically  from  the  clock  diagram,  but  this  method  is 
not  to  be  recommended  where  accuracy  is  sought. 

To  obtain  by  calculation  the  phase  angle  between  the  main 
current  7  and  the  voltage  E,  it  is  necessary  to  resolve  7  into  its 
components  parallel  to  and  in  quadrature  with  E. 


ALTERNATING-CURRENT  MACHINERY  53 

The  power  component  of  the  current  /i  is 


a      RJi      E       RI 
cos  0i=  —  —  =  —  X-   =Eg 


T) 

here  gi=  -^  is  called  the  conductance  of  circuit  1. 
*i  • 

Similarly  the  power  component  of  72  is 


J 

/2  COS  02=  ~ 


* 

The  "reactive"  or  "wattless"  component  of  the  current  /i  is 


where  61  =^  is  called  the  susceptance  of  circuit  1. 


Fig.  64.     Vector  Diagram  of  Current  Conditions 
for  Fig.  63 

Similarly  the  reactive  component  of  the  current  72  is 


Since  /  is  the  vector  sum  of  the  two  branch  currents 


The  power  component  of  the  resultant  current  is  the  sum  of  the 
power  components  of  the  two  branch  currents,  or 

I  cos  6=  Ii  cos  0i+  72  cos  02=  Eg1-{-Eg2=  E  (gi+gz) 
and  the  reactive  componert  of  I  is 

I  sin  0=  /i  sin  01+72  sin  02=  £&i+E62=  E  (&i+62) 


54  ALTERNATING-CURRENT  MACHINERY 

Therefore 

tan#= 


7cos# 

and  the  magnitude  of  the  vector  7  being  the  hypotenuse  of  a  right- 
angle  triangle,  having  a  base  7  cos  6,  and  an  altitude  7  sin  6,  is 


By  drawing  a  semicircle  on  OE  as  a  diameter  and  extending  the 
current  vectors  7i,  72,  and  7,  till  they  meet  the  semicircle  at  B,  A, 
and  C  in  Fig.  64,  some  important  relations  may  be  deduced.  Join 
the  points  A,  B,  and  C  with  E,  thus  completing  the  triangles 
OAE,  QBE,  and  OCE.  They  are  all  right-angle  triangles  because 
inscribed  in  semicircle. 

A  careful  study  of  Fig.  64  will  show  that  OA=RJ2}  AE=XZI2, 

(see  page  35),  and  that  <\[  OA2  +^AE2  =  OE  =  E  from  which  R2  = 

OA  AE  OE 

— ,  X 2=  -p-  ,  and  Z2  =  — . 
^2  72  72 


Similarly  OB=  RJl}  BE=  XJi,  and     OB  +  BE  =  0E=  E  and 


#7=  OC,  CE=XI,  andC2  +2  =  ^  in  which  #  is  called  the 
"equivalent  resistance"  of  the  total  circuit,  and  X  its  "equivalent  react- 
ance." In  other  words,  if  the  branched  circuits  were  replaced  by  a 
single  coil  having  a  resistance  equal  to  72  and  a  reactance  equal  to  X, 
the  coil  would  take  the  same  current  from  the  mains  as  before,  with 
the  same  angle  of  lag,  and  would  absorb  the  same  total  power.-  The 
impedance  of  this  single  "equivalent  coil"  is  not  the  sum  of  the 

F 
impedances  of  the  separate  coils,  but  is  —  . 

Example.  An  impedance  coil  having  a  resistance  of  19.05  ohms  and  a 
reactance  (at  a  frequency  of  60  cycles)  of  11  ohms  is  connected  in  parallel 
with  a  second  impedance  coil  having  a  resistance  of  22  ohms  and  a  reactance 
of  38.1  ohms  across  220-volt,  60-cycle  mains.  Find  (a)  the  current  in  each 
coil  and  the  total  (or  line)  current;  (b)  the  phase  relation  of  the  currents; 

(c)  the  power  factor  of  the  entire  circuit  and  the  power  expended  in  it;  and 

(d)  the  "equivalent"  resistance  and  reactance  of  the  circuit. 


ALTERNATING-CURRENT  MACHINERY  55 

Solu:ion.     (a)     To  find  the  currents,  we  have 

E          __  220  __  220 
=  Vi?T*f  "  V(19.Q5)  •+(!!)••  "  "2*  = 


E  220  220 

/  2  =     .  —  =     .  =  -—-  =  5  amperes 

V(22)2  +  (38.1)2       44 


and 


The  above  values  are  simply  the  magnitudes  of  the  branch  currents. 
Before  they  can  be  represented  in  a  clock  diagram,  their  directions  must  be 
found. 


el  =  -30° 

The  minus  sign  means  a  lagging  current. 

X        38  1 

tan  02  =  ---  =  -4-  =1.732,    or 
/to          22 

62  =  -  60° 

The  angle  of  phase  difference  between  7j  and  72  is 
02  -  Ol  =  -60°  -  (  -30)°  =  -  30° 

The  vector  currents  may  now  be  drawn  to  scale  as  is  done  in  Fig.  64. 
The  resultant  or  vector  sum  of  the  currents  7j  and  7Z  may  be  found  by  draw- 
ing the  diagonal  of  the  parallelogram  on  7X  and  72  as  sides.  The  main  current 
is  then  01. 

Its  magnitude  may  be  found  by  trigonometry;  thus 


,  72cos  (d2-d 


=  VlO2  +  52  +  (2  X  10  X  5  X  0.866)  =  V  211.6   =  14.55  amperes. 
To  find  the  phase  of  7,  we  have 


tan  0 


where  b>  -    "     " .  -    =°-0227;  6'-' "    «  °  °-01968 ; 


56  ALTERNATING-CURRENT  MACHINERY 

0.0227  +  0.01968   0.04238 
tan  9  =  0.0394+0.0114  -  5^5T 
or 

6  =  39°  53' 

which  is  the  angle  by  which  I  lags  behind  E. 

(c)  The  power  factor  of  coil  1  is  cos  6l — cos  30°  =0.86603   and  the 
power  expended  in  it  is  I\  flj  =  102X  19.05  =  1905  watts,  or  it  is  E2  g1  =  (220)z 
X  0.0394  =  1905  watts. 

The  power  factor  of  coil  2  is  cos  02  =  cos  60°  =0.5  and  the  power  ex- 
pended in  it  is  I\  R 2  =  (5) 2X 22  =  550  watts,  or  it  is  E*gt  =  (220) 2X 0.01 136  =  550 
watts. 

The  power  expended  in  coils  1  and  2  =1905  +  550  =  2455  watts. 

The  power  factor  of  the  entire  circuit  (two  branches)  is 
cos#=  cos  39°  53' =  0.76735 

and  the  power  expended  in  it  is 

El  cos  0=220X14.55X0.76735  =  2455  watts 

(d)  The  "equivalent  resistance"  R  is 

OC      220  cos  39°  53'       168.81 

T       -HiT"      •H55"11-6ohmB 

The  power  absorbed  in  R  is 

72#  =  (14.55)2X11.6=2455  watts 
The  "equivalent  reactance"  X  is 

CE      220  sin  39°  53 


/  14.55 


9.7  ohms 


The  current  /  lags  behind  the  voltage  E,  by  an  angle  6  given  by 

X  97 

tan  0  =  —  or  tan  d  =  —  -•  =0.836  from  which  0  =  -39°  53'  the  same  as  found 
K  ll.o 

above. 

Electromotive  Force  Losses  in  Transmission  Lines.  An  alter- 
nator of  which  the  electromotive  force  is  E,  delivers  current  over 
a  transmission  line  of  which  the  resistance  is  RI  and  the  reactance 
(inductance  reactance)  is  X\,  to  a  receiving  circuit  of  which  the 
resistance  is  R2  and  the  reactance  is  X2.  The  total  electromotive 


ALTERNATING-CURRENT  MACHINERY 


57 


force  used  to  overcome  the  resistance  and  the  reactance  of  the 
transmission  line  is  E\,  and  the  electromotive  force  between  the 


Fig.  65.     Diagram  of  E.  M.  F.  Losses  in  Transmission  Lines — Receiving  Circuit  Non-Reactive 

terminals  of  the  receiving  circuit  is  E2.  The  current  delivered  is 
7.  Then  the  general  relation  between  E,  EI,  and  E2  is  as  shown 
in  Fig.  62,  except  that  E2  is  usually  wry  much  larger  than  EI  in  value. 
There  are  three  interesting  and  simple  special  cases  of  electromo- 
tive force  losses  in  transmission  lines,  as  follows:* 

CASE  1.  When  the  receiving  circuit  is  non-reactive.  In  this 
case  the  electromotive  force  E2  between  the  terminals  of  the  re- 
ceiving circuit  is  in  phase  with  7,  the 
power  factor  of  the  receiving  circuit  is 
unity,  and  the  general  diagram  of  Fig. 
62  takes  the  form  shown  in  Fig.  65. 
The  total  electromotive  force  E1  con- 
sumed in  the  line  is  sometimes  called 
the  impedance  loss  or  drop  and  its  two 
components  7^7  and  XJ,  as  shown  in 
Fig.  65,  are  called  the  resistance  loss  and 
the  reactance  loss,  respectively.  Now, 
a  careful  inspection  of  Fig.  65  makes  it 
evident  that  the  numerical  difference  be- 
tween the  values  of  E  and  E2  is  very 
nearly  equal  to  the  resistance  loss  in 
the  line  7^7;  and  that  the  reactance  loss 
in  the  line  XJ  has  little  to  do  with  the 
difference  between  the  values  of  E  and 
E2.  Therefore,  when  the  receiving  circuit 
is  non-reactive,  the  difference  in  value  be- 
tween generator  electromotive  force  E  and 
receiver  electromotive  force  E2  is  sensibly 
equal  to  the  resistance  loss  of  electromotive  force  in  the  line,  and  sen- 
sibly independent  of  the  reactance  loss  of  electromotive  force  in  the  line. 

*This   discussion   applies   to   comparatively   short   lines,   ten  miles  or  less  in  length, 
inasmuch  ad  the  capacity  of  the  line  is  not  here  taken  into  account. 


Fig.  66.     E.  M.  F.  Losses  When 

the  Receiving  Circuit  is 

Highly  Reactive 


58 


ALTERNATING-CURRENT  MACHINERY 


CASE  2.  When  the  receiving  circuit  is  highly  reactive.  In  this 
case  the  electromotive  force  E2  between  the  receiving  circuit  ter- 
minals is  nearly  90  degrees  ahead  of  /  in  phase,  and  the  general 
diagram  of  Fig.  62  takes  the  form  shown  in  Fig.  66.  A  careful 
inspection  of  Fig.  66  makes  it  evident  that  the  difference  in  value 
of  E  and  E2  is  very  nearly  equal  to  the  reactance  loss  in  the  line 
XJ;  and  that  the  resistance  loss  in  the  line  RJ  has  little  to  do 
with  the  difference  between  the  values  of  E  and  E2.  Therefore, 
when  the  receiving  circuit  is  highly  reactive,  the  numerical  difference 
in  value  between  generator  electromotive  force  E  and  receiver  electro- 
motive force  E2  is  sensibly  equal  to  the  re- 
actance loss  of  electromotive  force  in  the 
line,  and  sensibly  independent  of  the  resist- 
ance loss  of  electromotive  force  in  the  line. 
CASE  3.  When  the  receiving  circuit 
has  large  capacity  reactance.  In  this  case 
the  electromotive  force  E%  between  the 
receiving  circuit  terminals  is  nearly  90  de- 
grees behind  7  in  phase,  and  the  general 
diagram  of  Fig.  62  takes  the  form  sliown 
in  Fig.  67.  A  careful  inspection  of  Fig. 
67  makes  it  evident  that  the  difference 
between  the  values  of  E  and  E2  is  very 
nearly  equal  to  the  reactance  loss  in  the 
line  XJ,  and  that  this  reactance  loss  is 
added  to  the  generator  electromotive  force 
E  to  give  the  receiver  electromotive  force 
E%.  Inspection  of  Fig.  67  shows  further- 
more that  the  resistance  loss  in  the  line 
RJ  has  little  to  do  with  the  difference  in  value  of  E  and  E2.  There- 
fore, when  the  receiving  circuit  has  a  high  capacity  reactance,  the  re- 
actance loss  in  the  line  is  sensibly  equal  to  the  rise  in  value  of  the  elec- 
tromotive force  between  generator  and  receiver,  and  this  rise  in  value 
is  sensibly  independent  of  the  resistance  loss  of  electromotive  force 
in  the  line.  It  is  somewhat  confusing  to  speak  of  the  electromotive 
force  XJ  as  reactance  loss  when  the  receiving  circuit  has  a  high 
capacity  reactance;  it  would  be  better  in  this  case  to  speak  of  XJ 
as  the  reactance  gain  of  electromotive  force  in  the  line. 


/E, 


Fig.  67.     E.  M.  F.  Losses  with 
Large  Capacity  Reactance 


ALTERNATING-CURRENT  MACHINERY  59 


Electromotive  Force  Losses  'in  Alternator  Armatures.  Let  E  be 
the  total  induced  electromotive  force  in  the  armature  of  an  alterna- 
tor. A  portion  EI  of  this  electromotive  force  is  used  to  overcome 
the  resistance  RI  and  the  reactance  Xi  of  the  armature;  and  the 
remainder  E2  is  available  at  the  terminals  of  the  alternator  for 
producing  current  in  the  outside  circuit,  of  which  the  resistance  is  R2 
and  the  reactance  is  X2.  The  general  relation  between  E,  Ev  aneL.Ea 
is  as  shown  in  Fig.  62,  except  that  E2  is  usually  wry  much  larger  than  EI 
in  value.  There  are  three  interesting  and  simple  special  cases  of  elec- 
tromotive force  losses  in  alternator  armatures,  as  follows: 

CASE  1.  When  the  receiving  circuit  is  non-reactive.  In  this 
case  the  electromotive  force  E2  between  the  terminals  of  the  alter- 
nator is  in  phase  with  the  current  I  delivered  by  the  machine,  and 
the  general  diagram  of  Fig.  62  takes  the  form  shown  in  Fig.  65, 
from  which  it  is  evident  that  when  the  receiving  circuit  is  non- 
reactive,  the  numerical  difference  in  value  between  the  total  induced 
electromotive  force  E  and  the  terminal  electromotive  force  E2 
of  the  machine  is  sensibly  equal  to  the  resistance  loss  of  electro- 
motive force  Ril  in  the  armature,  and  sensibly  independent  of  the 
reactance  loss  of  electromotive  force  XJ  in  the  armature. 

CASE  2.  When  the  receiving  circuit  is  highly  reactive.  In  this 
case  the  electromotive  force  E2  between  the  terminals  of  the  alter- 
nator is  nearly  90  degrees  ahead  of  I  in  phase,  and  the  general  diagram 
of  Fig.  62  takes  the  form  shown  in  Fig.  66,  from  which  it  is  evident 
that  when  the  receiving  circuit  is  highly  reactive  the  numerical 
difference  in  value  between  the  total  induced  electromotive  force  E 
and  the  terminal  electromotive  force  E2  of  the  machine  is  sensibly 
equal  to  the  reactance  loss  of  electromotive  force  XJ  in  the  arma- 
ture, and  sensibly  independent  of  the  resistance  loss  of  electro- 
motive force  Ril  in  the  armature. 

CASE  3.  When  the  receiving  circuit  has  large  capacity  react- 
ance. In  this  case  the  electromotive  force  E2  between  the  ter- 
minals of  the  alternator  is  nearly  90  degrees  behind  I  in  phase,  and  the 
general  diagram  of  Fig.  62  takes  the  form  shown  in  Fig.  67,  from 
which  it  is  evident  that  when  the  receiving  circuit  has  a  high  capacity 
reactance  the  difference  in  value  between  E  and  E2  is  sensibly  equal 
to  XJ,  E2  being  larger  than  E,  and  sensibly  independent  of  the 
resistance  loss  of  electromotive  force  RJ  in  the  armature. 


60  ALTERNATING-CURRENT  MACHINERY 

MEASURING  INSTRUMENTS 

Electrical  measuring  instruments  may  be  divided  into  three 
classes,  as  follows: 

(a)  Indicating  instruments  which  give  the  value  of  an  elec- 
trical quantity  at  the  time  of  observation,  or  which  may  be  so 
manipulated  as  to  give  this  value. 

(b)  Integrating  instruments   which   combine  the   element   of 
time  with  the  element  of  electrical  quantity.    For  instance,  a  watt- 
hour  meter  gives  a  reading  proportional  to  the  product  of  the  average 
power  in  watts  and  the  time  in  hours,  or  in  other  words,  it  measures 
the  electrical  energy  expended  in  the  circuit  to  which  it  is  connected ; 
it  is  not,  therefore,  a  power  meter. 

(c)  Recording  instruments  which  trace  a  curve  or  other  graphic 
record  showing  the  variation  of  some  electrical  quantity,  such  as 
voltage,  with  time.     It  should  be  carefully  noted  that  the  name 
"recording  wattmeter"  is  very  commonly  but  incorrectly  applied 
to  the  integrating  watt-hour  meter.    The  latter  is  not  a  recording 
instrument,  nor  is  it  a  wattmeter.     A  true  recording  wattmeter 
would  draw  a  curve  showing  the  variation  of  the  watts  with  time. 

Electrical  instruments  may  be  also  classified  according  to  con- 
struction and  method  of  use  into:  switchboard,  portable,  and  semi- 
portable  or  laboratory  types. 

INDICATING  INSTRUMENTS 

Indicating  electrical  instruments  may  be  divided  into  three 
groups,  as  follows:  (1)  Those  adapted  for  direct  currents  only,  a 
consideration  of  which  is  beyond  the  scope  of  this  text;  (2)  those 
adapted  for  both  direct-  and  alternating-current  circuits,  which 
class  comprises  hot-wire,  electrostatic,  and  electromagnetic  instru- 
ments, and  electrodynamometers ;  (3)  instruments  operating  only  on 
alternating  current,  which  depend  upon  the  interaction  of  induced 
and  inducing  currents,  and  are  usually  described  as  induction  instru- 
ments. 

Hot=Wire  Ammeter  and  Voltmeter.*  Instruments  of  the  hot- 
wire type  depend  upon  the  expansion  of  a  stretched  wire  when  heated 


*A11  voltmeters  except  fthe  electrostatic  voltmeter  are  essentially  ammeters;  that  is. 
the  electromotive  force  to  be  measured  produces  a  current  which  actuates  the  instrument, 
The  scale  over  which  the  pointer  moves,  may  be  arranged  to  indicate  either  the  value  of 
the  current  flowing  through  the  instrument,  or  the  value  of  the  electromotive  force  acting 
between  the  terminals  of  the  instrument. 


ALTERNATING-CURRENT  MACHINERY  61 


by  the  passage  of  a  current,  which  actuates  a  pointer  moving  over 
a  divided  scale.  These  instruments  are  adapted  for  both  direct  and 
alternating  current  for,  when  calibrated  by  continuous  currents  or 
electromotive  forces,  they  indicate  effective  values  of  alternating  cur- 
rents or  electromotive  forces.  This  may  easily  be  proved  true  by 
considering  an  alternating  current  and  a  continuous  current  C  which 
give  the  same  reading.  These  currents  generate  heat  in  the  wire- at 
the  same  average  rate,  which  is  C2R  for  the  continuous  current,  and 
average  i9XR  for  the  alternating  current,^  being  the  instantaneous 
value  of  the  alternating  current.  Therefore,  C2R=  average  i2XR;  or 
C2=  average  i2;  or  C=  V  average  iz. 

The  proof  for  electromotive  forces  is  similar  to  this  proof  for 
currents. 

Recent  instruments  of  the  hot-wire  type  have  a  working  wire 
from  3 1  to  8  inches  in  length,  of  small  size  for  voltmeters  and  of  larger 
size  for  ammeters.  The  voltmeters  have  a  non-inductive  resistance 
connected  in  series,  and  the  ammeters  usually  have  the  working  wire 
connected  in  parallel  with  several  sections  in  order  to  reduce  the 
required  drop  of  voltage  in  the  shunt. 

Hot-wire  instruments  are  comparatively  little  used  in  this 
country  in  practical  work,  their  disadvantages  being  relatively  large 
power  consumption,  uncertainty  of  zero  (on  the  scale),  and  errors 
due  to  change  of  surrounding  temperature  and  to  heating  when 
left  long  in  circuit;  furthermore,  to  secure  sensibility,  the  working 
wire  must  be  operated  at  a  fairly  high  temperature  and  may  thus  be 
easily  damaged  by  sudden  overloads  which  in  other  types  of  in- 
struments would  hardly  do  more  damage  than  the  bending  of  a 
pointer. 

The  advantages  of  hot-wire  instruments  which  cause  its  con- 
tinued use  for  certain  classes  of  work  are  its  independence  of  fre- 
quency, wave  form,  and  stray  magnetic  fields;  the  fact  that  it  may 
be  calibrated  on  direct  current;  and  the  fact  that  shunts  may 
be  used  with  the  ammeter  on  alternating  current.  For  use  in  labora- 
tories with  unusual  frequencies  or  wave  forms,  and  where  there  are 
facilities  for  calibration  on  direct  current,  the  hot-wire  type  has 
marked  advantages.  It  should  be  noted,  however,  that  for  very  high 
frequencies,  such  as  those  employed  in  wireless  telegraphy,  shunted 
hot-wire  ammeters  are  not  reliable.  This  is  due  to  the  fact  that  for 


62  ALTERNATING-CURRENT  MACHINERY 

such  high-frequency  currents  the  effective  resistance  of  the  shunt — 
including  the  so-called  "skin  effect" — is  much  greater  than  for  direct 
current,  and  that  the  current  in  the  shunt  will  lag  more  behind  the 
electromotive  force  than  the  current  in  the  working  wire.  For  these 
extreme  frequencies,  therefore,  it  is  necessary  to  use  a  hot-wire 
ammeter  so  constructed  as  to  permit  the  whole  current  to  pass 
through  the  working  wire  which  shall  have  its  effective  resistance 
at  the  frequency  used  practically  equal  to  its  resistance  for  direct 
current,  thus  eliminating  the  shunt  altogether. 

Fig.  68  is  a  general  view  of  the  hot-wire  voltmeter  of  the  Roller- 
Smith  Company,  and  Fig.  69  is  a  diagram  of  its  essential  mechanism. 
In  Fig.  69,  a  wire  a,  called  the  working  wire,  because  it  alone  carries 


Fig.  68.     General    View  of  a  Hot-wire 
Voltmeter — Roller-Smith  Company 

current,  is  fastened  at  one  end  to  a  plate  c,  passed  around  a  pulley 
d  secured  to  a  shaft  e,  and  its  free  end  brought  back  again  and  me- 
chanically attached  to,  though  electrically  insulated  from,  the  same 
plate  c.  The  wires  a  and  b  are  kept  under  tension  by  the  spring  / 
attached  to  the  plate  c,  which  is  being  constantly  pulled  in  a  direction 
at  right  angles  to  the  axis  of  the  shaft  e,  and  is  so  guided  that  it  can 
be  moved  in  that  one  direction  only.  To  the  shaft  e  is  secured  an 
arm  g  which  is  forked  at  its  lower  end  and  counterweighted  at  its 
upper  end.  Between  the  forked  ends  of  the  arm  g  is  another  shaft 
h  on  which  there  is  a  small  pulley  and  to  which  is  attached  the  light 


ALTERNATING-CURRENT  MACHINERY 


63 


pointer  i;  a  fine  silk  fiber  passes  around  the  pulley  and  is  secured  to  the 
ends  of  the  fork  arms  which  are  springy  and  keep  the  silk  fiber  taut. 

The  current  to  be  measured  passes  through  the  wire  a  only, 
entering  and  leaving  as  shown  by  the  arrow  heads  on  the  terminals 
near  c  and  d .    When  a  is  heated  by  the  current  it  expands,  causing 
its  tension  to  be  less  than  the  tension  of  b.   The  result  is  that  the  pulley 
d  is  rotated  in  a  clockwise  direction  until  the  tensions  in  a  and  b  ^r 
again  balanced.   When  the  pulley  d  is  rotated,  g  is  moved  to  the  left 
and  this  causes  the  silk  fiber  to  rotate 
the  shaft  h  and  the  pointer  i.     From 
this  construction  and  the  ratio  of  the 
lever  arms,  it  is  evident  that  a  very 
slight  elongation  of  the  wire  a  suffices 
to  produce  a  considerable  movement 
of  the  pointer  i. 

One  of  the  objections  to  hot-wire 
instruments,  viz,  that  the  working 
wire  is  affected  by  changes  in  the 
temperature  of  the  air  thereby  intro- 
ducing an  error  in  the  measurements, 
is  successfully  overcome  in  this  make 
of  instrument  by  simple  compensation. 
Thus,  if  the  temperature  of  the  sur- 
rounding air  changes,  the  wires  a  and 
b  are  affected  alike,  both  either  con- 
tracting or  expanding  by  the  same 

amount,  which  causes  a  movement  of  the  plate  c  back  or  forth  in 
its  path,  but  without  any  tendency  to  rotate  the  pulley  d. 

The  entire  moving  system  illustrated  in  Fig.  69  is  mounted  on 
a  single  base  plate  and,  by  means  of  a  lever  projecting  through  the 
instrument  case,  may  be  rotated  slightly  about  a  heavy  shaft  whose 
axis  is  in  line  with  the  axis  of  the  pointer  shaft  h.  The  scale  with 
its  support  being  stationary,  this  device  permits  of  correction  for  a 
bent  pointer,  or  adjustment  to  zero  on  the  scale  without  interfering 
with  the  mechanism  in  any  way. 

While  the  hot-wire  instruments  of  the  Roller-Smith  Company 
are  nearly  dead-beat,  an  auxiliary  damping  device  is  usually  fur- 
nished consisting  of  an  aluminum  disk  swinging  between  the  poles 


Fig.  69.     Diagram  of  Hot-wire 
Instrument 


64  ALTERNATING-CURRENT  MACHINERY 

of  a  stationary  permanent  magnet.  Hot-wire  voltmeters  are  specially 
suited  for  measuring  low  electromotive  forces  up  to  about  75  volts. 
The  switchboard  hot-wire  ammeters,  reading  from  25  up  to  1,000 
amperes,  have  separate  shunts  to  which  the  instrument  is  connected 
by  flexible  leads. 

Electrostatic  Voltmeter.  Instruments  of  the  electrostatic  type 
depend  upon  the  attraction  of  oppositely  charged  bodies,  and  repul- 
sion of  similarly  charged  ones.  Two  metallic  plates  connected  to 
the  terminals  of  a  battery,  or  any  other  source  of  electromotive  force, 
attract  each  other  with  a  force  which  is  exactly  proportional  to  the 
square  of  the  electromotive  force.  As  these  forces  are  relatively 
small,  instruments  of  this  type  are  not  w^ell  adapted  for  use  as  am- 
meters, and  it  is  difficult  to  construct  satisfactory  voltmeters  on  the 
electrostatic  principle  for  the  ordinary  low  voltages  of  110  to  220 
volts.  The  electrostatic  principle  is  especially  adapted  to  the  meas- 
urement of  high  voltages  from  about  10,000  up  to  200,000  volts. 
The  voltmeter  consists  essentially  of  a  fixed  metal  plate  and  a  mov- 
able plate  delicately  mounted  on  a  jewelled  pivot.  The  movable 
plate  carries  a  pointer,  which  plays  over  a  divided  scale.  The  electro- 
motive force  to  be  measured  is  connected  between  the  fixed  plate 
and  the  movable  plate,  and  the  electrical  attraction  between  the 
plates  causes  the  movable  plate  to  turn  about  its  supporting  pivot 
and  move  the  pointer.  Such  an  instrument)  when  calibrated  by  con- 
tinuous electromotive  force,  indicates  effective  values  of  alternating 
electromotive  force,  as  may  be  seen  from  the  following  discussion: 
A  given  deflection  of  the  movable  plate  depends  upon  a  definite 
average  or  constant  force  acting  between  the  two  plates.  The  force 
due  to  a  constant  electromotive  force  E  is  kE2,  that  is,  the  force  is 
proportional  to  E2;  and  the  average  force  due  to  an  alternating 
electromotive  force  e,  is  kX  average  e2.  If  these  electromotive  forces 
give  equal  deflections,  the  constant  force  kE2  must  be  equal  to  the 
average  force  kX average  e2;  that  is 

kE2  =  k  average  e2 
or 

E2  =  average  e2 
or 


V  average  e2=E 


ALTERNATING-CURRENT  MACHINERY 


65 


The  great  advantage  of  this  type  of  voltmeter  is  that  it  takes 
no  current  when  used  on  direct-current  circuits,  and  an  extremely 
small  current  when  used  on  alternating-current  circuits.  Its  other 
advantages,  like  those  of  the  hot-wire  instruments,  are  that  it  is 
independent  of  changes  in  frequency,  wave  form,  and  stray  mag- 
netic fields.  Furthermore,  for  very  high  voltages — up  to  several 
hundred  thousand  volts — the  construction  is  relatively  simple -and 
cheap,  and  no  auxiliary  "potential"  transformers  are  required  for 
reducing  the  voltage  to  be  measured.  It  has  the  disadvantage  of 
small  ratio  of  torque  to  weight  of  moving  parts,  so  that  errors  due 
to  friction  of  the  moving  element  are  relatively  large  and  difficult 
to  avoid.  For  this  reason  low-range  electrostatic  voltmeters  are 
often  made  with  the  moving  element  suspended  by  a  wire  or  strip 
instead  of  rotating  on  pivots. 


Fig.  70.    Diagram  of  Westinghouse  Electrostatic  Voltmeter 

Excepting  the  electrostatic  ground  detector,  which  is  essen- 
tially a  voltmeter,  this  type  of  instrument  has  as  yet  been  little 
used  in  commercial  work  outside  of  the  laboratory.  The  adoption, 
however,  of  increasingly  high  voltages  for  long-distance  transmission 
of  power  carries  with  it  a  demand  for  a  reliable  commercial  form  of 
this  instrument.  Great  progress  has  lately  been  made  towards 
putting  the  electrostatic  voltmeter  on  a  commercial  basis. 

Fig.  70  shows  a  diagrammatic  view  of  the  principle  and  arrange- 
ment of  parts  of  the  electrostatic  voltmeter  brought  out  by  the  West- 
inghouse Electric  Company.  Fig.  71  (a)  shows  the  moving  element. 

The  meter  element  consists  of  two  stationary  curved  aluminum 
plates  BI  and  J52,  between  which  is  suspended  a  movable  vane  MI  M% 


ALTERNATING-CURRENT  MACHINERY 


controlled  by  a  light  spiral  spring  so  adjusted  that  the  pointer  P 
remains  at  zero  with  no  voltage  on  the  meter,  and  gives  the  full  scale 
deflection  at  the  proper  voltage  for  which  the  instrument  is  designed. 
The  curved  plates  BI  and  B2  are  connected,  as  shown,  to  the  inner 
plates  of  the  condensers  Ci  and  C2,  and  so  arranged  with  respect  to 
MI  MI  that  when  a  voltage  difference  exists  between  the  terminals 
TI  and  T2,  the  induced  —  charge  on  MI  and  the  +  charge  on  M2 
being  attracted  by  the  +  charge  on  BI  and  the  —  charge  on  B2) 
respectively,  the  moving  element  MI  M2  rotates  counter-clockwise 
into  the  new  position  shown  by  the  dotted  lines  in  Fig.  70. 

The  condensers  C\  and  C2  are  in  series  with  the  other  parts  of 
the  instrument,  the  inner  plate  of  each  being  connected  to  the  fixed 


V 


(a)  Old  Form  of  Moving  Element  (b)  Modern  Moving  Element 

Fig.  71.     Westinghouse   Electrostatic   Voltmeter 

plates  BI  and  B2,  and  the  outer  plate  of  each  to  the  terminals  TI  and 
T2.  For  high  voltage  readings  the  instrument  is  connected  as  a 
shunt  across  one  of  the  two  (or  more)  condensers  in  series,  thus 
impressing  any  desired  fraction  of  the  total  voltage  upon  the  instru- 
ment terminals.  For  reading  lower  voltages,  one  or  more  of  the 
condensers  are  short-circuited,  thus  permitting  the  same  instrument 
to  be  used  over  a  wide  range.  Fig.  71  (b)  shows  a  modern  element. 
The  meter  is  placed  in  a  sheet-iron  tank  rilled  with  transformer 
oil.  This  is  necessary  because  oil  has  a  far  greater  dielectric  strength 
than  air,  and  in  this  meter  the  distance  between  the  parts,  between 


ALTERNATING-CURRENT  MACHINERY 


67 


which  there  is  a  high  voltage  difference,  is  less  than  the  distance  at 
which  arcing  across  in  air  would  occur.  The  oil  also  acts  to  dampen 
the  moving  element,  thus  making  the  instrument  dead-beat  and 
easy  to  read. 

The  electrostatic  ground  detector  is  a  modified  electrostatic  volt- 
meter. Its  essential  features  are  shown  in  Fig.  72.  Two  metal 
plates  A  and  B  are  connected  to  the  two  mains  a  and  b  (usually 
through  two  high  resistances  RR);  and  a  light  movable  metal  plate 
m,  suspended  between  A  and  B,  is  connected  to  ground  (usually 
through  a  high  resistance  R).  If  both  lines  are  equally  well  insulated 
from  ground,  the  plates  A  and  B  are  each  at  the  same  electrical 
pressure  or  potential,  attracting  the  plate  m  equally  so  that  it  hangs 
midway  between  them.  If  one  of  the  mains,  say  a,  is  grounded,  its 
pressure  or  potential  becomes  equal  to  the  potential  of  m,  so  that 
plate  A  no  longer  attracts  m,  and  the 
plate  m  is,  therefore,  pulled  to  the  right 
by  the  attraction  of  B,  the  movement 
being  indicated  by  a  pointer  p. 

The  same  results  may  be  accom- 
plished, when  the  plate  m  is  entirely 
insulated  from  the  ground,  by  having 
a  grounded  auxiliary  stationary  plate 
near  m. 

The  essential  features  and  mode  of 
connection  of  the  General  Electric  Com- 
pany's electrostatic  ground  detector  are 
shown  in  Fig.  73 ;  and  a  general  view  of 
the  instrument,  with  cover  removed,  is 
shown  in  Fig.  74. 

Electromagnetic  Ammeters  and  Volt= 

meters.  Instruments  of  this  type  depend  upon  the  action  of  a  coil  car- 
rying the  current  to  be  measured,  upon  one  or  more  pieces  of  soft  iron. 
Such  instruments  are  also  called  "moving-iron",  "soft-iron",  and 
"magnetic- vane"  types.  Instruments  of  this  general  type  have  been 
used  for  many  years,  the  earlier  ones  having  been  constructed  with 
heavy  soft  iron  cores  which  were  drawn  into  solenoids  energized  by  the 
current  to  be  measured.  The  pull  on  the  plunger  due  to  the  current 
was  balanced  by  a  weight  attached  to  a  lever  arm.  The  earlier  design 


Fig.  72.     Diagram  of  Essentials  of 
Electrostatic  Ground  Detector 


68 


ALTERNATING-CURRENT  MACHINERY 


sometimes  referred  to  as  the  "plunger "  type,  was  faulty,  and  has 
been  since  superseded  by  the  modern  designs  which  involve  the 


GROUND 


GROUND 


Fig.  73.     Connecting  Circuits  for  an  Electro- 
static Ground  Detector 

use  of  one  or  two  thin  vanes  of  soft  iron  mounted  on  a  pivoted  staff 
within  the  coil. 

The  soft-iron  type  of  instrument  is  calibrated  by  the  use  of 
direct  electromotive  forces  or  currents,  but  it  does  not  indicate 
accurately  effective  values  of  alternating  electromotive  forces,  or 


Fig.  74.     General  Electric   Ground   Detector 
with  Cover   Removed 


currents.     A  soft-iron  type  meter  (ammeter  or  voltmeter)  should 
be  calibrated,  using  alternating  current  of  the  same  frequency  as 


ALTERNATING-CURRENT  MACHINERY 


69 


that  for  which  the  meter  is  afterwards  to  be  used,  and  of  the  same 
wave  shape.  Thus,  if  an  instrument  of  the  electromagnetic  type 
is  to  be  used  as  an  ammeter  for  alternating  currents  of  a  given  wave 
shape  and  frequency,  it  should  be  calibrated  by  currents  of  this 
wave  shape  and  frequency,  these  currents,  for  the  purpose  of  the 
calibration,  being  measured  by  a  standard  alternating-current  am- 
meter, such  as  an  electrodynamometer. 

The  indications  of  an  instrument  of  the  electromagnetic  type, 
however,  do  not  vary  greatly  with  wave  shape  and  frequency,  and 
such  instruments  are  practically  correct  for  any  ordinary  wave 
shape  and  frequency. 

The  ammeters  are  but  slightly  affected  by  even  large  changes 
in  frequency;  the  voltmeters,  on  account  of  their  relatively  large 
inductance,  are  more  affected  than  the  ammeters.  The  error  in  the 


Fig.  75.     Mounted  Coil  of  a  Thomson  Inclined-Coil  Meter 

voltmeter  is  not  large  over  the  ordinary  range  of  frequencies,  and 
may  be  computed  for  extreme  frequencies  from  the  measured  values 
of  resistance  and  inductance.  A  valuable  feature  of  this  type  of 
instrument  is  its  very  small  temperature  coefficient.  They  are 
well  adapted  for  commercial  measurements  on  alternating  current 
circuits,  and  for  direct  current  when  only  approximate  results 
(within  2  or  3  per  cent)  are  needed.  The  electromagnetic  volt- 
meter or  ammeter  cannot  be  checked  accurately  on  direct  currents, 
as  even  the  average  of  reversed  readings  does  not  give  an  accurate 
test  of  the  performance  on  alternating  current.  This  is  due  to  the 
hysteresis  effect  in  the  soft-iron  vanes,  thereby  causing  the  instru- 
ment to  read  higher  for  decreasing  direct  currents  than  for  increas- 
ing values.  This  difference  may  amount  to  several  per  cent.  An- 
other advantage  of  these  instruments  is  their  moderate  price,  and 
simple  construction. 


70 


ALTERNATING-CURRENT  MACHINERY 


Thomson  Inclined-Coil  Meter.  The  Thomson  inclined-coil  meter 
of  the  General  Electric  Company  is  an  example  of  the  electromag- 
netic type.  The  working  parts  of  this  instrument  are  shown  in 
Fig.  75.  A  coil  A,  through  which  flows  the  current  to  be  measured, 
is  mounted  with  its  axis  inclined  as  shown.  A  vertical  staff  mounted 
in  jewel  bearings  and  controlled  by  a  hair  spring,  passes  through  the 
coil,  and  to  this  staff  are  fixed  a  pointer  b  and  a  vane  of  thin  soft 
sheet  iron  a.  This  vane  of  iron  is  mounted  obliquely  to  the  staff. 
When  the  pointer  is  at  the  zero  point  of  the  scale,  the  iron  vane  a 
lies  nearly  across  the  axis  of  the  coil;  and  when  a  current  passes 
through  the  coil,  the  vane  tends  to  turn  until  it  is  parallel  to  the 
axis  of  the  coil,  position  a'  in  Fig.  75,  thus  turning  the  staff  and 
moving  the  attached  pointer  over  the  calibrated  scale.  The  vane 


Fig.  76.     Thomson  Inclined-Coil 
Ammeter 


Fig.  77.     Working  Parts  of  Thomson 
Inclined-Coil  Ammeter 


tends  to  turn  into  a  position  which  makes  the  reluctance  of  the 
magnetic  circuit  a  minimum. 

Fig.  76  is  a  general  view  of  a  Thomson  inclined-coil  ammeter, 
and  Fig.  77  is  a  view  of  the  working  parts  of  the  instrument.  The 
structural  details  of  the  inclined-coil  voltmeter  are  identical  with 
those  of  the  ammeter,  except  that  the  voltmeter  has  fine  wire  in 
the  inclined  coil  and  usually  an  auxiliary  non-inductive  resistance 
in  series  with  the  inclined  coil. 

Roller-Smith  Repulsion  Ammeter.  Another  example  of  an  am- 
meter of  the  electromagnetic  type  using  the  repulsion  principle  is 
illustrated  in  Figs.  78,  79,  80,  which  give  a  plan,  sectional  elevation, 
and  detail,  respectively.  It  is  manufactured  by  the  Roller-Smith 
Company.  The  current  to  be  measured  is  led  by  cables  to  the 


ALTERNATING-CURRENT  MACHINERY 


71 


terminals  a  and  «i  and  thence  to  the  coil  through  heavy  copper 
straps  shown  in  dotted  outline  underneath  the  scale  in  Fig.  78. 
The  terminal  ai  is  thus  electrically  connected  to  the  brass  spool  m 
to  which  is  soldered  the  conducting  strip  j,  Fig.  79,  whose  carrying 
capacity  is  adapted  to  the  current  for  which  the  instrument  is  de- 
signed. The  conductor  j  is  wound  about  the  spool  upon  a  layer 
of  insulating  material  to  give  the  required  number  of  turns  "and 
is  finally  brought  to  the  post  r,  Fig.  78,  which  passes  down  through 


Fig.  78.     Plan  of  Roller-Smith  Repulsion  Ammeter 

the  insulating  bushing  /,  and  connects  underneath  with  the  copper 
strap  terminating  at  a.  The  same  method  of  winding  is  used  in 
the  case  of  voltmeters,  except  that  instead  of  the  copper  straps, 
flexible  leads  are  used  to  connect  the  spool  to  binding  posts  covered 
with  hard  rubber,  and  located  outside  of  the  case. 

The  moving  element  c  is  carried  and  supported  by  the  structure 
shown  at  g,  which  is  secured  to  the  brass  spool  by  the  two  hexagon- 
headed  bolts  as  shown  in  Fig.  78.  This  construction  enables  the 
moving  elements  as  well  as  the  coils  to  be  made  up  separately  as 
occasion  demands,  and  independently  of  the  rest  of  the  mechanism, 
and  thus  reduces  the  manufacturing  cost. 

The*  structure  g  consists  of  a  casting  into  which  is  inserted  a 
piece  of  brass  tubing,  having  a  solid  bottom,  for  holding  the  lower 


72 


ALTERNATING-CURRENT  MACHINERY 


jewel  screw.  The  side  of  the  tube  is  used  to  support  the  fixed 
magnetic  vane  b.  The  moving  iron  vane  c  is  fastened  to  the  staff  i, 
which  also  carries  the  pointer  e,  which  moves 
over  the  divided  scale.  The  vibrations  of  e  are 
dampened  by  the  vane  /,  Fig.  79,  which  is  forced 
to  move  in  the  nearly  air-tight  box  which  forms 
an  integral  part  of  the  casting.  The  controlling 
spring  d  is  adjusted  so  that  the  pointer  e  is 
normally  held  at  zero  on  the  scale.  If  the 
pointer  fails  to  return  to  zero,  it  may  be 
brought  there  by  means  of  the  lever  h,  which 
is  connected  to  a  button  on  the  outside  of  the 
case. 

The  action  of  the  instrument  is  as  follows: 
when  a  current  is  passed  through  the  coil  wind- 
ing j,  it  magnetizes  both  the  fixed  and  movable 
vanes  within  the  coil.     Both  vanes  being  made 
SECTION  A-B  °^  very  so^  ^ron>  are  similarly  magnetized  and, 

Fig.  79.  sectional  Elevation  therefore,  repel  each  other.    The  movable  vane 

of  Repulsion  Ammeter  .  .  i       i       •          T  • 

c  thus  rotates  in  a  clockwise  direction  carrying 
the  pointer  e  to  the  right  along  the  scale.  This  movement  of  the 
vane  is  opposed  by  the  action  of  the  controlling  spring  d  resulting 
in  the  pointer  coming  to  rest  at  a  point  depending  upon  the  strength 
of  the  current  passing  in  the  coil.  The  design  and  arrangement 
of  the  movable  and  fixed  soft  iron  vanes  is  such  as  to  give  the 
scale  shown  in  Fig.  78,  which  is  reproduced  from  an  actual  cali- 
brated scale.  The  pivots  of  the  moving  system  are  of  steel,  tem- 
pered and  highly  polished,  and  the  bearings  are  of  highly  polished 
sapphires. 

Electrodynamometers.    Of  the  four  types  of 
instruments    for  use  on  both  direct  and  alter- 
nating currents,  the  electrodynamometer   is  on 
the  whole  the   most  valuable.     This  instrument 
depends  upon   the  force  exerted  by  one  circuit 
Fof  R?puis?oenaAmmegterm  carrying  a  current  upon   another,  or  by  a  por- 
tion of  a  given  circuit  upon  another  portion  of 
the  same  circuit.     Instruments  of  this  type  usually  contain  one 
or  more  fixed  coils  which  set  up  a  magnetic  field  directly  proportional 


ALTERNATING-CURRENT  MACHINERY 


73 


to  the  strength  of  the  current  flowing  through  them.  Within  this 
field  is  arranged  a  moving  coil,  or  system  of  coils,  through  which 
current  may  be  passed.  If  the  two  setc 
of  coils  are  connected  in  series,  the  torque 
exerted  upon  the  moving  system,  for  a 
given  relative  position  of  the  coil  sys- 
tems, is  proportional  to  the  square  of  the 
current,  and  is  not  dependent  upon  the 
direction  of  the  current.  Such  an  instru- 
ment, therefore,  constitutes  an  ammeter 
which  is  equally  correct  on  direct  cur- 
rent, and  on  alternating  or  pulsating 
current  of  any  frequency  or  wave  form. 
The  value  of  this  type  consists  largely 
in  this  inherent  accuracy  on  these  differ- 
ent kinds  of  current.  Since  the  funda- 
mental electrical  standards  and  precision 


methods  of  testing  involve  direct  current  '  y  ' 


only,     the    electrodynamometer     type,  Fig.  €l.    sectional  Elevation  of  sie- 

calibrated   on   direct   current,    may   be 

used  as  a  precision  instrument  for  alternating  current. 

Electrodynamometer  Used  as  an  Ammeter.  The  electrodyna- 
mometer consists  of  a  fixed  coil  and  a  movable  coil  connected  in  series, 
through  both  of  which  the  current  to  be  measured  flows.  The 
current  causes  the  fixed  coil  to  exert  a  certain  force  upon  the  mova- 
ble ceil;  and  the  value  of  the  current  is  determined  (a)  by  observing 
the  angle  <$>  through  which  a  helical  spring 
must  be  twisted  by  hand  in  order  to  bal- 
ance the  above-mentioned  force;  or  (b) 
by  allowing  the  force  to  turn  the  mov- 
able coil,  thus  moving  a  pointer  over  a 
divided  scale.  In  the  Siemens  electro- 
dynamometer  method  (a)  is  used;  while  / 
in  many  commercial  forms  of  electrody-  x^ 
namometer  method  (b)  is  used. 

The  essential  features  of  the  Siemens 

electrodynamometer  are  shown  in  Figs.  81  and  82.     The  stationary 
coil  A  is  supported  by  a  clamp  attached  to  the  standard  S;  and 


Ele°tr°" 


74  ALTERNATING-CURRENT  MACHINERY 

the  movable  coil  B  is  hung  by  a  thread,  the  plane  of  coil  B  being 
at  right  angles  to  the  plane  of  coil  A.  The  terminals  of  the  mov- 
able coil  dip  into  cups  of  mercury  a  a,  and  the  current  to  be 
measured  is  sent  through  both  coils  in  series.  The  force  acting 
between  the  coils  is  balanced  by  carefully  turning  the  torsion  head 
c  by  hand,  thus  twisting  a  helical  spring  b,  one  end  of  which  is 
attached  to  the  coil  B  and  the  other  to  the  torsion  head  c.  The 
observed  angle  of  twist  necessary  to  bring  the  swinging  coil  to  its 
zero  position,  is  read  off  by  means  of  the  pointer  d  and  the  grad- 
uated scale  e.  The  pointer  /  attached  to  the  coil  shows  when  it 
has  been  brought  to  its  zero  position.  The  observed  angle  of 
twist  of  the  helical  spring  affords  a  measure  of  the  force  acting 
between  the  coils,  and  the  current  is  proportional  to  the  square 
root  of  this  angle  of  twist.  That  is 

I=kV~$  (28) 

in  which  7  is  the  effective  value  of  the  alternating  current;  (f>  is 
the  observed  angle  of  twist  of  the  helical  spring  b;  and  k  is  a  con- 
stant called  the  reduction  factor  of  the  instrument. 

For  example,  a  twist  of  220°  is  required  to  balance  the  force 
due  to  18.8  amperes  in  a  certain  Siemens  electrodynamometer; 
the  reduction  factor  k  of  the  instrument  is,  therefore,  by  equation 
(28),  equal  to  18.8  amperes  -f-  i/220~,  or  1.267.  A  certain  current 
to  be  i  measured  requires  a  twist  of  the  torsion  head  of  165°,  hence 
the  value  of  the  current  is  equal  to  1.267  X  1/165,  or  16.28  amperes. 

The  electrodynamometer,  when  standardized  by  direct  currents, 
indicates  effective  values  of  alternating  currents;  thus,  a  given  de- 
flection of  the  suspended  coil  depends  upon  a  definite  average  or 
constant  force  acting  between  the  coils.  The  constant  force  due 
to  a  constant  current  C  is  kC2  (proportional  to  C2) ;  and  the  average 
force  due  to  an  alternating  current  is  &X average  i2;  so  that  if  these 
currents  give  equal  deflections,  we  have  kC2=kXaveragei2;  or  C2— 
average  i2;  or  C  =  V average  i2. 

Electrodynamometer  Used  as  a  Voltmeter.  When  used  as  a  volt- 
meter the  coils  of  the  electrodynamometer  are  made  of  fine  wire, 
and  an  auxiliary  non-inductive  resistance  is  usually  connected  in 
series  with  the  coils.  When  the  inductance  of  the  electrodynamometer 
coils  is  small,  such  an  instrument,  when  calibrated  by  continuous  electro- 


ALTERNATING-CURRENT  MACHINERY  75 

motive  forces,  indicates  effective  values  of  alternating  electromotive 
forces. 

When  it  is  certain  that  the  inductance  of  an  electrodyna- 
mometer  is  negligibly  small,  the  instrument  may  be  used  in  precise 
alternating  electromotive  force  measurements.  In  order  to  deter- 
mine this  it  will  be  necessary  to  find  the  inductance  error  of  the  electro- 
dynamometer  when  used  as  a  voltmeter.  An  electrodynamometer 
which  has  been  calibrated  by  continuous  electromotive  forces  in- 
dicates less  than  the  effective  value  of  an  alternating  electromotive 
force.  Let  E  be  the  reading  of  an  electrodynamometer  voltmeter 
when  an  alternating  electromotive  force  (harmonic),  of  which  the 
effective  value  is  E,  is  connected  to  its  terminals.  That  is,  E  is  the 
continuous  electromotive  force  which  gives  the  same  deflection  as  E; 
and,  since  E  gives  the  same  deflection  as  E,  it  follows  that  the  effective 
current  produced  by  E  is  equal  to  the  continuous  current  produced 
by  E;  that  is 

JL-  E 

R 


in  which  R  is  the  total  resistance  of  the  instrument;  L  its  inductance; 
and  a>  =  2nf,  where  /  is  the  frequency  of  the  alternating  electromotive 
force.  Solving  the  above  equation  for  E,  we  have 


That  is,  the  reading  of  the  instrument  must  be  multiplied  by  the 


factor   -  -  -  to  give  the  true  effective  value  of  a  harmonic 
H 

alternating  electromotive  force. 

Induction  Instruments.  Instruments  suitable  for  alternating 
current  only,  depend  upon  the  interaction  of  inducing  and  induced 
currents,  and  are  commonly  described  as  "induction"  instruments. 
They  are  usually  designed  with  a  laminated  iron  core  around  which 
one  or  more  coils  of  wire  are  wound.  An  alternating  magnetic  flux 
is  produced  in  the  air  gap  of  this  core  when  current  passes  through 
the  coils.  The  effect  of  a  rotating  magnetic  field  is  secured,  either 
by  having  currents  differing  in  phase  pass  through  two  groups  of 
coils,  or  by  using  a  single  inducing  coil  with  fixed  copper  plates  or 


76 


ALTERNATING-CURRENT  MACHINERY 


bands  in  which  induced  currents  are  generated.  The  resultant  action 
of  the  flux  due  to  the  coil  and  that  due  to  the  induced  currents  in 
the  fixed  copper  pieces,  produces  the  effect  of  a  rotating  magnetic 
field.  An  aluminum  disk  or  drum  pivoted  in  the  field  tends  to  rotate 
with  the  rotating  field.  The  motion  of  the  drum  or  disk  is  opposed 
by  a  suitable  spring  in  the  case  of  indicating  instruments,  whereas 
retarding  magnets  and  registering  mechanism  to  read  the  total 
quantity  that  has  passed  through  the  meter  are  provided  in  the  case 
of  integrating  instruments,  such  as  the  watt-hour  meter. 

Indicating  or  direct-deflection  induction  meters  are  made  in 
portable  form  for  commercial  testing,  but  are  more  generally  used 
for  switchboard  instruments,  including  ammeters,  voltmeters,  watt- 
meters, frequency  meters,  and  power  factor  meters. 

The  advantages  possessed  by  port- 
able induction  indicating  instruments 
are  that  they  are  not  sensitive  to 
external  stray  fields,  that  they  have 
long  (over  300°)  open  scales,  that  the 
moving  element  is  simple,  light,  and 
strong,  and  has  no  windings  and, 
therefore,  needs  no  provision  for  lead- 
ing current  in  and  out. 

The  disadvantages  of  this  type  of 
meter  are  that  its  readings  tend  to 
vary  more  or  less  with  changes  in  fre- 
quency and  temperature,  although  in 
instruments  of  the  latest  design  these 
errors  have  been  greatly  reduced. 

These  instrument's  should  be  calibrated  under  conditions  (espe- 
cially frequency)  as  nearly  as  possible  like  those  under  which  they 
are  to  be  used. 

The  construction  and  principle  of  induction  indicating  instru- 
ments is  illustrated  by  the  "Type  F"  induction  voltmeter  made  by 
the  Westinghouse  Electric  Company  for  switchboard  service,  shown 
in  Fig.  83  with  its  case  and  dial  removed.  The  view  shows  the  fine 
wire  primary  coils  of  a  current  transformer  wound  about  the  two 
legs  of  an  inverted  U-shaped  structure  of  laminated  iron.  The 
primary  coils  are  connected  in  series  with  a  high-resistance  wire  of 


Fig.  83.     Westinghouse  Induction 
Voltmeter 


ALTERNATING-CURRENT  MACHINERY 


77 


zero  temperature  coefficient,  the  object  of  which  is  to  make  the 
readings  of  the  voltmeter  more  nearly  independent  of  frequency. 
The  secondary  coil  of  the  current  transformer  is  wound  directly 
under  the  primary  coils  and  connected  to  the  few  turns  of  coarse 
wire  marked  "auxiliary  coils,"  Fig.  83,  on  the  pole  pieces.  The 
entire  secondary  circuit  forms  a  closed  winding.  The  poles  constitute 
in  effect  a  two-phase  bipolar  rotating  field  produced  by  two  magnetic 
fluxes  which  are  nearly  at  right  angles  to  each  other.  One  of  these 
fluxes  is  due  to  the  auxiliary  coils  carrying  the  secondary  current 
and  the  other  to  the  magnetizing  component  of  the  primary  current. 
To  vary  the  capacity  of  the  meter,  the  number  of  primary  turns 
and  the  size  of  wire,  only,  need  to  be  changed.  In  the  air  gap  between 
the  poles  is  the  moving  element  which  is  a  very  light  aluminum  drum 
mounted  on  a  shaft  having  highly 
polished  and  hardened  pivots  rest- 
ing in  polished  sapphire  jewels. 
This  shaft  also  carries  the  indi- 
cating pointer  and  control  spring 
which  opposes  the  torque  by  the 
moving  element. 

Type  F  voltmeters  are  made 
for  circuits  ranging  in  voltage  from 
150  up  to  750  volts.  For  higher 
voltages,  voltage  transformers  are 
required. 

Fig.  84  is  a  general  view  of  an 
induction  indicating  ammeter  also 
made  by  the  Westinghouse  Electric 

Company  for  switchboard  service.  It  is  identical  in  principle  with  the 
voltmeter  above  described,  practically  the  only  difference  being  in  the 
primary  coils  of  the  current  transformer  which  have  fewer  turns  of 
larger  wire.  They  are  wound  for  5  amperes  and  can  be  connected  to 
circuits  where  smaller  currents  are  to  be  measured  and  the  voltage  is 
less  than  1,000  volts.  When  the  current  exceeds  5  amperes,  series  or 
current  transformers  are  used,  having  their  secondaries  wound  for 
5  amperes. 

Wattmeter.    A  good  wattmeter  is  the  standard  instrument  for 
measuring  power  in  alternating-current  circuits.     The  wattmeter 


Fig.  84.     Westinghouse  Induction 
Indicating    Ammeter 


78  ALTERNATING-CURRENT  MACHINERY 

is  an  electrodynamometer,  of  which  one  coil  a,  Fig.  85,  made  of  fine 
wire,  is  connected  to  the  terminals  of  the  circuit  CC,  in  which  the 
power  to  be  measured  is  expended.  The  other  coil  b,  made  of  large 
wire,  is  connected  in  series  with  CC,  as  shown.  The  fine-wire  coil 
a  is  movable,  and  carries  the  pointer  which  indicates  on  a  divided 
scale  the  watts  expended  in  CC. 

Such  an  instrument  ivhen  calibrated  with 
continuous  current  and  electromotive  force 
indicates  power  accurately  when  used  with 
alternating  currents,  provided  the  induc- 
tance of  the  circuit  ar  is  small.  This  is  true 
whatever  the  wave  shape  of  the  electromo- 
tive force  or  current,  and  whatever  the 

ma/'n N|          character    of    the  receiving    circuit  CC. 

Fig.  ss.    Diagram  of  an  Aiterna-     Thus  the  circuit  CC  may  have  any  react- 
Ciwaittme°tnetrain  ance,  as  may  be  seen  from  the  following 

discussion :  A  given  deflection  of  the  mov- 
able coil  a  depends  upon  a  certain  average  or  constant  force  action  be- 
tween the  coils.  Consider  a  continuous  electromotive  force  E,  which 

produces  a  current  —  in  a,  and  a  current  C  in  CC  and  b.  The  force 
action  between  the  coils  is  proportional  to  the  product  of  the  currents 

in  a  and  b;  that  is,  the  force  action  is  kX  —  X  C,  where  &  is  a  constant. 

r 

Consider  an^alternating  electromotive  force  of  which  the  instan- 

/> 

taneous  value  is  e;  this  produces  a  current — through  a  (provided  the  in- 
ductance of  a  is  zero),  and  a  current  i  in  CC  and  b.  The  instantaneous 

p 

force  action  between  the  coils  is  kX  ~Xi;  and  the  average  force  action 

k 

is  — X  average  ei.     If  this  alternating  electromotive  force  gives  the 

same  deflection  as  the  continuous  electromotive  force,  then 

k  k 

—  X  aver  age  ei=  —  EC 
or  r  r 

average  ei=  EC 

that  is,  the  given  deflection  indicates  the  same  power  whether  the 
currents  are  alternating  or  direct. 


ALTERNATING-CURRENT  MACHINERY 


79 


Power  Factor.  Let  P  be  the  true  power  delivered  to  the  circuit 
CC  as  measured  by  a  wattmeter;  let  E  be  the  effective  electromotive 
force  between  the  terminals  of  CC  as  measured  by  an  alternating- 
current  voltmeter;  and  let  I  be  the  effective  current  flowing  in  CC 
as  measured  by  an  alternating-current  ammeter. 

P  . 

Then  the  ratio  —  is  called  the  power  factor  of  the  circuit  CC. 
hil 

Examples.  1.  The  primary  coil  of  a  certain  5-kilowatt  transformer 
with  secondary  coil  open-circuited  takes  0.14  ampere  from  1,000-volt  mains; 
and  the  power  as  measured  by  a  wattmeter  is  100  watts.  The  power  factor, 
therefore,  is 

100  watts 
1,000  volts  X  0.14  ampere 

2.  One  of  the  stator  circuits  of  a  polyphase  induction  motor  running 
unloaded  has  a  power  factor  of  0.6.  It  takes  2  amperes  from  200-volt  mains. 
The  true  power,  as  would  be  indicated  by  a  wattmeter,  is 

P  -  200  volts  X  2  amperes  X  0.6  =  240  watts 

Compensated  Wattmeter.  If  a  wattmeter  is  connected  as  shown 
in  Fig.  85,  it  measures  the  power  delivered  to  the  circuit  CC  plus 
the  power  consumed  in  heating  the  circuit  ar;  hence  the  wattmeter 
reading  is  greater  than  the  power  delivered  to  the  circuit  CC. 

Again,  if  a  wattmeter  is  connected  as  shown  in  Fig.  86,  it  meas- 
ures the  power  delivered  to  the  circuit  CC 
plus  the  power  consumed  in  heating  the 
coil  b;  hence,  in  this  case  also,  the  watt- 
meter reading  is  greater  than  the  power 
delivered  to  the  circuit  CC. 

The  compensated  wattmeter,  which  is 
of  the  direct-deflection  type,  manufac- 
tured by  the  Weston  Electrical  Instru- 
ment Company,  is  designed  to  eliminate 
the  above-mentioned  sources  of  error  in 
the  following  manner,  the  connections 
being  shown  in  Fig.  85.  Let  C  be  the  current  in  CC,  and  let  a  be 
the  current  in  a  and  r.  Then  the  current  in  b  is  C-\-a,  and  the  force 
acting  upon  the  movable  coil  is  proportional  to  the  product  a 
(C4-a),  instead  of  being  proportional  to  the  product  aC.  In  the 
compensated  wattmeter,  the  wire  leading  over  to  the  coil  a,  con- 


ma/n 


ma/n 


Fig.  86.     Diagram  of  Alternating 

Circuit  Containing  Wattmeter 

Improperly  Connected 


80  ALTERNATING-CURRENT  MACHINERY 

nected  as  shown,  is  laid  alongside  each  and  every  turn  of  wire  in 
coil  b.  Then  current  C-\-a  flows  down  through  b;  current  a  flows 
back  alongside  the  wire  of  coil  b;  and  the  result  is  the  same  as  if  the 
current  a  were  subtracted  from  the  current  (7+  a,  so  far  as  the  mag- 
netic action  of  the  coil  b  is  concerned. 

Portable  Torsion  Wattmeter.  A  portable  wattmeter  of  the  torsion 
type  manufactured  by  the  Roller-Smith  Company,  Figs.  87  and  88, 
is  of  the  electrodynamometer  type  but  without  the  mercury  cups  of 


Fig.  87.     Roller-Smith  Portable  Torsion  Wattmeter 

the  Siemens  instrument,  the  current  being  led  into  and  out  of  the 
fine  wire  movable  (voltage)  coil  through  the  springs  s  and  Si.  Like 
the  Siemens  instrument,  it  is  operated  by  rotating  a  torsion  head 
through  the  nut  k,  Fig.  88,  until  the  tension  of  the  controlling  springs 
s  and  Si  balances  the  torque  of  the  movable  element  a.  The  attain- 
ment of  a  balance  between  the  two  opposing  forces  is  indicated 
when  the  needle  h  attached  to  the  moving  element  registers  with  the 
zero  mark  on  the  scale.  The  amount  of  torsion  required  to  bring 
the  needle  h  back  to  zero  is  indicated  by  the  scale  reading  of  the 
index  pointer  e,  which  is  secured  to  the  torsion  head  j. 

Referring  to  Fig.  88,  the  terminals  for  the  current  (fixed)  coils 
c  are  shown  at  p,  and  those  for  the  voltage  (movable)  coil  a  at  p\. 


ALTERNATING-CURRENT  MACHINERY 


81 


The  current  coils  a  are  clamped  to  and  supported  by  a  metal  frame 
which  is  built  up  in  sections,  each  section  being  carefully  screwed 
and  locked  together,  and  the  joints  insulated  to  prevent  the  forma- 
tion of  eddy  currents  in  the  frame.  The  moving  or  voltage  coil  a  is 
clamped  to  and  supported  by  the  staff  g.  The  lower  controlling 
spring  *i,  is  fastened  to  an  arm  which  is  rotated  by  the  torsion  head 
j  by  means  of  the  rubber  covered  nut  k  projecting  through  the  glass 
top  of  the  wattmeter.  The  spring  Si  has  its  outer  convolution  secured 
to  a  lever  which  projects  through  the  side  of  the  case.  By  adjusting 
this  lever,  correction  to  zero  may  be  made.  The  staff  g  is  of  brass 
with  hardened  and  polished  pivots  resting  in  polished  sapphire  bear- 
ings. The  improved  construction  of  the  torsion  head  ball  bearing 
shown  at  /  requires  only  the  slightest  turning  effort,  and  unevenness 


Fig.  88.     Vertical  Section  of  Roller-Smith 
Portable  Torsion  Wattmeter 

in  manipulation  is  obviated.  Vibrations  of  the  movable  element  are 
dampened  by  the  vane  which  is  enclosed  in  the  nearly  air-tight  box  b. 
On  the  side  opposite  the  dampening  vane  and  just  counter- 
balancing its  weight  is  the  needle  h  which  passes  up  through  a  slot 
in  the  brass  scale  pan  and  indicates  the  position  of  the  moving  system 
on  the  dial.  The  scales  of  these  instruments  are  about  10  inches  long 
covering  an  arc  of  over  300  degrees,  a  feature  which  is  favorable  to 
accuracy  of  reading.  Another  advantage  of  this  type  of  wattmeter 
is  that  the  movable  element  is  always  brought  back  to  the  same 
position  with  respect  to  the  field  produced  by  the  fixed  coils  when 
readings  are  being  taken.  This  feature  eliminates  the  error  caused 
by  a  varying  angle  between  the  movable  and  fixed  coils,  and  is  of 
special  importance  when  measuring  power  in  circuits  having  different 
power  factors. 


82  ALTERNATING-CURRENT  MACHINERY 

A  short  scale  shown  at  the  top  of  Fig.  87  is  drawn  on  either  side 
of  the  zero  line  for  the  needle  h.  It  is  used  when  measuring  the  power 
of  a  fluctuating  load  and  indicates  the  watts  to  be  added  or  sub- 
tracted from  the  watts  shown  by  the  index  pointer  e. 

These  torsion-type  wattmeters  are  made  for  standard  maximum 
voltages  of  150,  300,  and  600  volts.  For  higher  voltages,  multipliers 
(large  non-inductive  resistances)  to  be  connected  in  series  with 
the  voltage  coil  of  the  wattmeter  are  furnished.  These  wattmeters 
are  made  either  for  one  current  or  for  two  different  currents.  The 
two-current  instruments  are  furnished  with  binding  posts  arranged 
for  connecting  the  current  coils  either  in  series  or  in  parallel. 

A  torsion-type  polyphase  wattmeter,  also  made  by  the  Roller- 
Smith  Company,  consists  of  two  instrument  mechanisms  super- 
imposed and  having  the  two  movable  coils  secured  to  a  common 
staff;  it  gives  accurate  readings  on  two-  or  three-phase  circuits, 
whether  the  load  is  balanced  or  not,  and  is  equally  useful  on  single- 
phase  and  on  three-wire  circuits. 

INTEGRATING   INSTRUMENTS 

The  watt-hour  meter  is  an  instrument  for  summing  up  (inte- 
grating) the  total  electrical  work  or  energy  expended  in  a  circuit 
in  a  given  time.  It  registers  on  suitable  dials  the  energy  in  terms 
of  kilowatt  hours.  These  meters  are  often  referred  to  as  "recording 
wattmeters,"  though  incorrectly,  for  they  are  not  wattmeters,  nor 
do  they  record.  Commercial  watt-hour  meters  may  be  divided 
into  two  classes,  according  to  the  principle  they  employ:  (a)  elec- 
trodynamometer  type,  (b)  induction  type.  They  are  all  motor 
meters.  Some  are  adapted  to  operate  on  direct  current  only,  some 
on  alternating  current  only  (the  induction  type),  and  others  on 
either  kind  of  current. 

Thomson  Watt=Hour  Meter.  The  Thomson  watt-hour  meter, 
made  by  the  General  Electric  Company,  is  a  small  electric  motor 
without  iron,  the  field  and  armature  coils  of  which  constitute  an 
electrodynamometer.  It  is  furnished  with  a  small  commutator  and 
brushes,  and  though  it  may  be  used  on  both  direct-  and  alternating- 
current  circuits,  it  is  principally  used  for  direct-current  work.  For 
alternating  currents  the  induction  type  of  meter  is  usually  employed. 

The  field  coils  BB  of  this  motor,  Fig.  89,  are  connected  in 


ALTERNATING-CURRENT  MACHINERY 


83 


series  with  the  circuit  CC,  in  which  the  work  to  be  measured  is 
expended.  The  armature  A,  together  with  an  auxiliary  non- 
inductive  resistance  R,  is  connected  between  the  terminals  of  the 
circuit  CC,  as  shown.  Current  is  led  into  the  armature  by  means 
of  the  brushes  dd  pressing  on  a  small  silver  commutator  e. 

The  stationary  and  movable  coils  are  connected  to  the  mains  and 
receiving  circuit,  that  is,  to  the  load,  exactly  as  are  the  stationary  and 
movable  coils  of  the  indicating  wattmeter;  and  the  torque  which 
the  stationary  (field)  coils  exert  upon  the  movable  coil  (armature)  is 
proportional  to  the  watts  delivered;  that  is,  is  proportional  to  the 
rate  at  which  energy  is  being  delivered  to  the  receiving  circuit. 

The  instrument  is  so  constructed  that  the  speed  of  its  armature 
is  proportional  to  the  torque  that  drives  it.  Therefore,  the  rate  of 


Fig.  89.     Diagrammatic  View  of  Thomson 
Watt-Hour  Meter 

turning  of  the  armature  is  proportional  to  the  rate  at  which  energy 
is  delivered  to  the  circuit  CC.  Hence,  the  total  number  of  revolu- 
tions turned  by  the  armature  in  a  given  time  is  proportional  to  the  total 
energy  expended  in  the  circuit  CC. 

To  make  the  armature  speed  proportional  to  the  driving  torque, 
the  armature  is  mounted  so  as  to  be  free,  as  nearly  as  possible,  from 
ordinary  friction;  and  an  aluminum  disk  /,  Fig.  89,  is  mounted  on 
the  armature  spindle  so  as  to  rotate  between  the  poles  of  permanent 
steel  magnets  MM.  To  drive  such  a  disk  requires  a  driving  torque 
proportional  to  its  speed. 


84 


ALTERNATING-CURRENT  MACHINERY 


Starting  Coil.  In  the  above  discussion  it  is  assumed  that  the 
.orque  which  opposes  the  motion  of  the  armature  A,  Fig.  89,  is 
proportional  to  the  speed  of  the  armature.  In  fact,  however,  this 
opposing  torque  may  be  considered  as  consisting  of  two  parts:  (1)  the 
torque  required  to  overcome  friction;  and  (2)  the  torque  required  to 
overcome  the  damping  action  of  the  magnets  on  the  aluminum  disk. 
The  first  part  of  the  torque  may  be  taken  to  be  approximately  con- 
stant, while  the  second  part  is  accurately  proportional  to  the  speed. 
Therefore,  an  arrangement  for  exerting  on  the  armature  a  constant 
torque,  sufficient  to  overcome  friction,  would  largely  eliminate 
errors  due  to  friction.  This  is  accomplished  in  the  Thomson  meter 
by  supplementing  the  field  coils  B,  Fig.  89,  with  an  auxiliary  field 


Fig.  90.    General  View  of  Thomson  Watt-Hour  Meter 

coil,  called  a  starting  coil,  connected  in  the  armature  circuit.  So  long 
as  the  electromotive  force  between  the  mains  does  not  vary,  the 
current  in  the  starting  coil  is  constant  and  it,  therefore,  exerts  a 
constant  torque  upon  the  armature.  If,  however,  the  electromotive 
force  between  the  mains  varies,  the  torque,  due  to  the  starting  coil, 
varies  with  the  square  of  the  electromotive  force. 

Example.  A  certain  watt-hour  meter  will  not  run,  even  if  started  by 
a  slight  impulse  from  the  hand,  until  the  delivered  power  reaches  37.5  watts; 
that  is,  the  running  friction  of  the  watt-hour  meter  is  equal  to  the  driving 
torque  produced  by  37.5  watts  of  delivered  power.  This  meter  is  adjusted 
(by  moving  the  damping  magnets)  so  as  to  read  correctly  when  the  delivered 


ALTERNATING-CURRENT  MACHINERY 


85 


s^wwvnL 

b          Res/sfance  LA 

— I        1 

;  Line 


Load 


Fig.  91.     Thomson  Watt-Hour  Meter  Con- 
nected to  a  Two- Wire  Current  Supply 


power  is  500  watts.  What  will  the  instrument  indicate  when  run  for  four 
hours  with  constant  delivery  of  200  watts  of  power? 

Solution.  Express  driving  torque  in  terms  of  watts  of  power  delivered  to 
the  receiving  circuit,  since  the  driving 
torque  is  proportional  to  the  watts 
delivered.  Let  us  express  speed  in 
terms  of  watt  hours  indicated  by  the 
dials  per  hour.  Now,  the  speed  is 
proportional  to  that  part  of  the  driv- 
ing torque  which  is  used  to  over- 
come the  retarding  action  of  the 
damping  magnets.  In  the  problem 
under  consideration,  the  total  driving 
torques  are  500  watts  and  200  watts, 
respectively;  and  since  the  running 
friction  absorbs  37£  watts  of  torque, 
the  torques  available  for  overcoming 
the  retarding  action  of  the  damping 
magnets  are,  respectively,  (500—37^) 
watts  and  (200 -37£)  watts.  The 
speed  in  the  first  case  is  500  watt 

hours  per  hour.  Let  x  be  the  speed  (watt  hours  indicated  per  hour)  in  the 
second  case.  Then 

(500 -37*)  :  (200 -37£)  :  :  500  :  x 

This  gives  a  value  of  175.6  watt  hours  per  hour  for  x,  so  that  the  watt  hours 
recorded  in  four  hours  run  will  be  4X175.6  watt  hours  per  hour,  or  702.4 
watt  hours.  The  actual  total  of  watt  hours  delivered  is  of  course  4  hours  X  200 
watts,  or  800  watt  hours. 

The  effect  of  running  friction  in  a  watt-hour  meter  is  to  cause  the  in- 
strument to  read  low  for  an  amount 
of  delivered  power  that  is  less  than 
the  delivered  power  for  which  the 
damping  magnets  are  adjusted  to 
give  a  correct  reading. 

Fig.  90  shows  a  general  view 
of  a  Thomson  watt-hour  meter. 
Fig.  91  shows  the  connections 
of  a  Thomson  watt-hour  meter 
to  two-wire  supply  mains,  and 
Fig.  92  shows  the  connections  to 
three-wire  supply  mains. 

Induction  Watt=hour  Meter. 
The  single-phase  induction  watt- 
hour  meter  has  many  advantages  over  the  type  using  commutator 
and  brushes,  and  in  one  form  or  another  is  very  extensively  used  in 


Fig.  92.     Thomson  Watt-Hour  Meter  Con- 
nected to  a  Three-Wire  Current  Supply 


86  ALTERNATING-CURRENT  MACHINERY 

measuring  electrical  energy.  In  principle  and  operation  it  is  merely 
a  small  single-phase  induction  motor  having  stationary  shunt  and 
series  windings  so  related  and  arranged  with  respect  to  the  lami- 
nated iron  core  as  to  produce  a  rotating  field,  which  acting  upon  a 
closed  movable  secondary  causes  it  to  rotate.  The  shunt  or  volt- 
age winding,  consisting  of  a  large  number  of  turns  of  fine  wire  wound 
on  a  laminated  iron  core,  has  a  high  inductance  and,  therefore,  the 
current  in  it  lags  about  90  degrees  behind  the  impressed  line  volt- 
age. The  series,  or  current,  winding  composed  of  but  a  few  turns 
of  coarse  wire,  has  a  very  low  resistance  and  inductance  and,  there- 
fore, the  current  in  it  will  be  in  phase  with  the  impressed  voltage, 
if  the  circuit  in  which  the  en  ,rgy  is  being  measured,  is  non-inductive. 
Thus  the  field  produced  by  the  shunt  winding  will  lag  approxi- 
mately 90  degrees  behind  that  of  the  series  winding,  on  a  non- 
inductive  load.  When,  therefore,  the  alternating  current  in  the 
series  coil  has  its  maximum  value,  the  current  in  the  shunt  coil 
has  its  minimum  value.  If  it  were  not  for  the  iron  core  loss  and 
copper  (PR)  loss  in  the  shunt  coil,  the  angle  of  lag  would  be  exactly 
90°.  In  commercial  meters  provision  is  made  for  making  this 
angle  90°,  whatever  the  power  factor  of  the  circuit  may  be.  The 
strength  of  the  rotating  field  flux,  that  is,  the  resultant  produced 
by  the  series  and  shunt  coils  together,  assuming  a  lag  of  exactly 
90°,  is  proportional  to  the  product  of  the  currents  in  the  two  coils 
and,  therefore,  proportional  to  the  product  of  the  current  and  the 
voltage  in  the  circuit  being  measured.  At  any  power  factor  less 
than  unity,  the  resultant  field  flux  is  proportional  to  this  product 
multiplied  by  the  sine  of  the  angle  of  phase  difference  between  the 
two  meter  currents.  If  the  current  in  the  voltage  coil  is  exactly 
in  quadrature  with  the  voltage  of  the  metered  circuit,  at  any  power 
factor  the  sine  of  the  angle  of  phase  difference  between  the  currents 
in  the  meter  circuits  will  be  equal  to  the  cosine  of  the  angular  dis- 
placement between  the  current  and  voltage  in  the  metered  circuit. 
Under  these  conditions,  therefore,  the  strength  of  the  shifting  field 
is  proportional  also  to  the  power  factor  of  the  circuit.  In  other 
words,  the  strength  of  the  rotating  field  is  proportional  to  the  product 
of  the  volts,  amperes,  and  power  factor  and  is,  therefore,  a  measure  of 
the  actual  power. 

Single-Phase.    A    view    of   a    typical    single-phase    watt-hour 


ALTERNATING-CURRENT  MACHINERY 


87 


meter  of  the  induction  type  manufactured  by  the  Westinghouse 
Electric  Company  is  shown  with  its  cover  removed  in  Fig.  93,  and 
with  its  case  removed  in  Fig.  94.  The  shape  of  its  electromagnetic 
circuit  is  shown  diagrammatically  in  Fig.  95.  That  the  resultant 
field  produced  by  the  shunt  and  series  coils  is  actually  a  shifting  or 
rotating  one  may  be  seen  by  a  careful  study  of  Figs.  95  and  96. 


TERMINALS 


TERMINAL 
BUSHINGS 


DIAL  AND 
REGISTERING 
MECHANISM  \ 


RETARDING 
MAGNETS 


MAGNET 
CLAMP 


DISK 


.  TERMINAL 
INSULATING 
PAD 


POWER  FACTOR 
ADJUSTMENT 


MAGNET 
CLAMPING 
SCREWS  A 


MAGNET 
SUPPORTING  AND 
ADJUSTING 
FRAME 


;LAMPING 

>CREWSB 


LIGHT  LOAD 
ADJUSTING 
SCREWS  C 


Fig.  93.     Typical  Single-Phase  Westinghouse  Watt-Hour  Meter 


The  dotted  lines  in  Fig.  95  show  the  main  paths  of  the  magnetic 
flux  produced  by  the  two  field  windings,  but  the  direction  of  the 
fluxes  are  constantly  reversing  owing  to  the  alternations  of  the  cur- 
rent in  the  coils.  Referring  to  the  shunt  and  series  pole  tips  by  the 
letters  used  in  Fig.  95  the  relation  of  the  field  fluxes  at  each  quarter 
period  (cycle)  may  be  followed  with  the  help  of  Fig.  96.  The  signs 
+  and  —  represent  the  instantaneous  values  of  the  poles  indicated. 


88 


ALTERNATING-CURRENT  MACHINERY 


Thus,  at  one  instant  the  shunt  pole  tips  A,  C,  and  A'  are  maximum 
+>  — ,  and  +,  respectively,  because  the  instantaneous  value  of  the 
current  is  maximum,  while  the  value  of  the  series  flux  is  zero.  At 
J  period  later  the  shunt  current  is  zero,  giving  zero  magnetic  poten- 
tial at  the  pole  tips,  while  the  series  current  has  reached  a  maximum 
value,  giving  maximum  —  and  +  at  the  pole  tips  B  and  Z).  At 
the  next  J  period  the  shunt  current  is  again  maximum  but  in  a  di- 
rection opposite  to  what  it  was  at  the  beginning,  making  the  pole  tips 
A,  C,  and  A'  —,  +,  and  — ,  respectively,  while  the  series  current 


TOP  BEARING 
SC-REW 


.SERIES 
COILS 


REGISTERING 
MECHANISM 


DISK 


POWER  FACTOR 
ADJUSTMENT 


LIGHT  LOAD 

ADJUSTMENT 

LOOPS 


RETARDfNG 

MAGNET 


MOUNTING 
FRAME 


MAGNET 
CLAMPING 
SCREWS  A 


SHUNT 
COIL 


LAMINATED 
ELECTRO  MAGNET 
CORE 


Fig.  94.     WestinghouselSingle-Phase  Watt-Hour  Meter  with  Case  Removed 

j-gain  is  zero.  Continuing,  the  other  relations  of  +  and  —  poles 
shown  in  Fig.  96  are  obtained.  It  will  ,be  observed  from  the  table 
that  both  the  +  and  —  signs  move  constantly  in  the  direction 
from  A'  to  A,  indicating  a  shifting  of  the  field  in  this  direction,  the 
process  being  repeated  during  each  cycle. 

The  losses  in  the  meter  coils  are  very  low,  particularly  when 
compared  with  the  high  torque  developed.     The  potential   coils 


ALTERNATING-CURRENT  MACHINERY 


89 


K  °  / 

^ 

J--T  — 
^___|_l 

^r 

"/"   \ 

**  - 

x^ 

.-' 

SHI/NT  ELEMENT 

Fig.  95.     Diagram  of  Electromagnetic  Circuit 
of  Westinghouse  Single-Phase  Watt- 
Hour  Meter 


have  a  loss  of  1.4  to  1.8  watts 
per  phase  and  the  series  coil  pro- 
duces a  drop  of  only  0.4  volt  at 
full  load. 

The  moving  element  consists 
of  a  thin  aluminum  disk  which 
rotates  about  a  vertical  shaft 
in  the  air  gap  in  which  the  rota- 
ting magnetic  flux  is  produced. 
The  disk  acts  like  the  squirrel- 
cage  rotor  (armature)  of  an  in- 
duction motor.  Currents  are  in- 
duced in  it  which  combine  with 
the  rotating  field  to  produce  a 
torque  proportional  to  the  power 
in  the  circuit.  This  torque  is 
counterbalanced  by  that  due  to 

the  generator  action  of  the  permanent  retarding  magnets  in  inducing 
currents  in  the  disk,  so  that  the  speed  is  exactly  proportional  to 
the  torque.  In  this  meter  the  disk  performs  two  distinct  functions. 
It  serves  not  only  as  the 
armature  of  a  motor,  but 
also  as  the  retarding  or  gen- 
erator element,  the  disk  be- 
ing rotated  by  the  meter 
field  at  one  edge  while  it  is 
retarded  by  the  permanent 
magnet  field  at  the  opposite 
edge. 

The  power  factor  adjust- 
ment, Fig.  94,  consists  of  a 
short-circuited  loop  enclos- 
ing part  or  all  of  the  shunt 


START 


/  PERIOD 


/  PERIOD        - 


PERIOD 


•  fi/LL  PERIOD, 


O          - 


O  -' 


Y 


field  flux.      The  flux  induces 

a  current  in  the  loop  which, 

acting  with  the  current  in       Fig.  96. 

the  shunt  coil,  produces  a 

slightly  lagging  field.    By   shifting   the   position,   or  changing  the 


Diagram  of  Relation  betwen  Field  Fluxes 
for  One  Cycle 


90 


ALTERNATING-CURRENT  MACHINERY 


resistance,  of  this  loop,  the  lag  may  be  so  adjusted  that  the  shunt 
field  flux  is  exactly  90  degrees  behind  the  voltage  in  phase.  This  ad- 
justment, however,  makes  the  meter  correct  at  or  near  one  particular 
frequency  only.  This  type  of  meter  should  be  calibrated  and  adjusted 
at  or  near  the  frequency  at  which  it  is  to  be  used.  When  such  a  meter 
has  been  adjusted  to  read  correctly  at  or  near  some  particular 
frequency,  such  as  60  cycles,  it  will  read  too  low  at  higher  frequencies, 
as  is  shown  by  the  lowest  of  the  curves  in  Fig.  97. 


••/o/ 


99 


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LOAD  CUffVE AT  CONSTANT  VOLTAGE  AMD  FREQUENCY 


%  f?£GISTff  AT/ON 
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L  TAGE  CL/ffVE  A  T  C0/ySTAMT  L  OAD  AMD  rffEQUEMCY 

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Fig.  97.     Curves  Showing  Per  Cent  of  Variation  in  Registration  of  Single-Phase  Watt- 
Hour  Meter  for  Variation  in  Load,  Voltage,  and  Frequency 


The  three  curves  of  Fig.  97  show  graphically  the  per  cent  of  varia- 
tion in  registration  of  these  meters  due  to  any  per  cent  of  variation 
in  load,  voltage,  and  frequency. 

All  meters  of  this  type  have  some  initial  bearing  friction  which 
if  not  compensated  for  would  require  a  portion  of  the  driving  torque 
of  the  meter  to  overcome  it,  and  would  result  in  no  rotation  at  very 
small  loads,  and  in  general  the  meter  registration  would  be  too  low 
at  light  loads.  Compensation  for  initial  friction  especially  when  the 
meter  is  running  on  light  loads  is,  therefore,  important  and, consists 
in  providing  a  constant  torque  adjustable  to  the  exact  value  of  the 


ALTERNATING-CURRENT  MACHINERY 


91 


frictional  torque,  and  one  entirely  independent  of  the  load  on  the 
meter.  The  compensating  torque,  moreover,  must  not  be  so  great 
as  to  cause  the  moving  element  to  rotate  or  "creep"  when  no  current 
is  passing  in  the  series  coil. 


LIGHT  LOAD 
ADJUSTING 
SCREWS  C 


POWER  FACTOR^ 
ADJUSTMENT 
foR  UPPER  ELEMENT 


.LAMPING 

SCREWS  B 


UPPER 

RETARDING 

MAGNETS 


MAGNET 
SUPPORTING 

.ANDAOJUSTINfi 

FRAME 


5PPER 
ISK 


TERMINAL 
BUSHINGS 


LOWER 
DISK 


MAGNET 
CLAMR 


TERMINALS 


MOUNTING 
FRAME 


MAGNET 
CLAMPING 

SCREWSA.' 


CASE 


LFig. 


LIGHT  LOAD 
ADJUSTING 
SCREWS  C' 


Westinghouse  "Type  C"  Polyphase  Induction  Watt-Hour  Meter 


Compensation  for  meter  friction,  or  "light  load  adjustment,"  is 
made  by  slightly  unbalancing  the  magnetic  fluxes  in  the  two  limbs 
of  the  shunt  magnetic  circuit.  To  do  this  a  short-circuited  loop  is 
placed  in  each  air  gap  and  by  moving  either  of  the  two  screws  shown 
at  C,  Vig.  93,  and  at  C",  Fig.  98,  the  position  of  the  loops  may  be 
changed,  so  that  one  loop  will  enclose  and  choke  back  more  of  the 


(,.)watt  Hrs.  MULTIPLY     BY    Watt  HrsQ, 


No.l=  1,1 11,100 


No.2=  999,900 


O          1,000.000      100X)00       10,000          £j 


No.3= 1,000,100 


ipoo.ooo   IOQOOO      10,000 


10,000.000 


ipoo 


o. 


No.  4=  9,999,500 


ID 


1,000,000       100.000      10.000 


01 


6, 

10,000,000 


1,000 


ip 


o. 


No.6=  99,700 


1,000,000        100POO       10,000 


10,000,000. 

o 


O 


Na7=  9,912,100 


1,000.000    looooo     iaooo 


10,000,000 

.0 


!£. 
ipoo 


No.8=  9,928,000 


IPOO.OOO      100,000       10,000 


2 
3-1  U3 


10,000,000 


ipoo 


No.9= 9,918,100 


ipoo.ooo    100,000       10,000 


10.0C  0,000 


ipoo 


pj 


KP,5  -909,100  No.lO=9t928,300 

Fig.  99.     Illustrative  Watt-Hour  Meter  Readings 


fluv 


ALTERNATING-CURRENT  MACHINERY  93 


flux  than  the  other  loop,  and  thus  produce  a  slight  torque  in  the 
disk.  This  slight  torque  depends  only  on  the  voltage  which  is  prac- 
tically constant,  and  is  entirely  independent  of  the  load  current. 

Polyphase.  The  polyphase  induction  watt-hour  meter  is  vir- 
tually two  independent  single-phase  meter  elements  having  a  common 
shaft.  Each  meter  element  exerts  a  torque  on  the  shaft  and  the 
combined  torque  is,  therefore,  proportional  to  the  sum  of  the  elec- 
trical energy  in  the  two  phases  being  measured,  regardless  of  the 
phase  relation  between  the  phases.  These  polyphase  meters  whefT 
properly  connected  indicate  the  true  energy  in  a  polyphase  system, 
whatever  the  degree  of  unbalance  between  the  phases  may  be,  and 
for  any  power  factor. 

Fig.  98  shows  a  Westinghouse  "Type  C"  polyphase  induction 
watt-hour  meter  with  covers  removed.  It  is  evident  that  it  is  prac- 
tically two  single-phase  watt-hour  meters  combined  to  make  one 
instrument. 

Directions  for  Reading  Watt=Hour  Meter  Dials.  To  read 
correctly  the  dial  of  a  watt-hour  meter,  some  care  is  necessary.  The 
figures  marked  under  or  over  a  dial  (1,000,  10,000,  etc.),  are  the 
amounts  recorded  by  a  complete  revolution  of  the  hand;  therefore, 
one  division  on  a  dial  indicates  one-tenth  of  the  amount  indicated 
above  or  below.  A  complete  revolution  of  the  first  hand  (the  one 
to  the  extreme  right)  in  No.  6,  Fig.  99,  for  example,  indicates 
1,000,  and  moves  the  second  hand  one  division  of  the  second  dial. 
The  first  hand,  in  the  position  given,  indicates  700 — not  7,000. 

In  deciding  on  the  reading  of  a  hand,  the  hand  before  it  (to 
the  right)  must  be  consulted.  Unless  the  hand  before  it  has  reached 
or  passed  the  0,  or  in  other  words  completed  a  revolution,  the  other 
has  not  completed  the  division  on  which  it  may  appear  to  rest.  For 
this  reason,  ease  and  rapidity  are  gained  by  reading  a  meter  from 
right  to  left.  For  example,  in  No.  2,  the  first  dial  (the  extreme  right) 
reads  900.  The  second  apparently  indicates  0;  but,  since  the  first 
has  not  completed  its  revolution,  but  indicates  only  9,  the  second 
cannot  have  completed  its  division;  hence  the  second  dial  also  in- 
dicates 9.  The  same  is  true  of  the  hand  of  the  third  dial;  the  second, 
being  9,  has  not  quite  completed  its  revolution;  so  the  third  has  not 
completed  its  division  and,  therefore,  we  again  have  9.  The  same 
holds  true  of  the  hand  of  the  fourth  dial.  The  last  hand  (the  extreme 


94 


ALTERNATING-CURRENT  MACHINERY 


left)  appears  to  rest  on  1 ;  but  since  the  fourth  is  only  9,  the  last 
has  not  completed  its  division  and,  therefore,  indicates  0.  Putting 
the  figures  down  from  right  to  left,  the  total  reading  is  999,900, 
though  one  might  erroneously  read  1,999,900,  making  a  mistake  of 
1,000,000  units. 

***]  The  hands  sometimes  become  slightly  misplaced,  as  shown  in 
Nos.  8,  9,  and  10.  In  No.  8  we  have  on  the  first  dial  (the  extreme 
right)  0;  we,  therefore,  put  down  three  zeros  thus,  000.  The  hand 
of  the  second  dial  is  misplaced ;  for  inasmuch  as  the  first  registers  0, 


Fig. 


/ff     eo    e4    ^8  32    36    40    44     48   5£    SG    6O    64     68 
THOUSANDS  OF  WA  TT-HOVKS 

100.     Graphical  Chart  for  Calculating  Customers'  Bills 


the  second  should  rest  exactly  on  a  division;  therefore,  we  know  that 
it  should  have  reached  8,  making  8,000.  The  remaining  hands  are 
correct,  and  make  a  total  of  9,928,000. 

In  No.  9  the  second  hand  is  misplaced;  for  since  the  first  in- 
dicates 1,  the  second  should  have  just  passed  a  division,  and,  as  it 
is  nearest  to  the  8,  we  know  it  should  have  just  passed  that  figure. 
The  remaining  three  hands  are  approximately  correct.  The  total 
reading  is  9,918,100. 

In  No.  10  the  second  hand  is  bekmd  its  correct  position.  The 
total  indication  is  9,928,300. 


ALTERNATING-CURRENT  MACHINERY 


95 


By  carefully  following  these  directions  one  will  find  little  difficulty 
in  reading  a  meter  even  if  the  hands  become  misplaced.  The  above 
directions  apply  to  watt-hour  meters  of  all  kinds — induction  type 
as  well  as  Thomson  meters. 

Calculations  of  Customers*  Bills.  The  consumption  of  energy 
in  watt  hours  being  known,  the  exact  amount  of  the  bill  can  at 
once  be  read  from  a  price  chart,  one  of  which  is  shown  in  Fig.  100. 
Dollars  and  cents  are  read  from  the  vertical  line  at  the  left,  while 
the  horizontal  line  at  the  bottom  of  the  chart  indicates  thousands 
of  watt  hours.  To  determine  the  amount  to  be  charged  for,  say 
28,000  watt  hours  at  12J  cents  per  1,000  watt  hours,  follow  the 


Fig.   101.     High  Potential  Testing  Transformer 
of  the  General  Electric  Company 

vertical  line  marked  28  to  its  intersection  with  the  diagonal  line 
marked  12J  cents.  From  the  intersection  of  these  two  lines,  follow 
the  horizontal  line  to  the  left,  and  find  $3.50,  which  is  the  amount 
of  the  bill  for  28,000  watt  hours  at  12J  cents  per  1,000  watt  hours. 
In  practice,  of  course,  the  price  chart  would  be  drawn  to  a  large  scale 
for  accuracy  in  reading. 

Spark  Gauge.  The  high  electromotive  forces  used  in  break- 
down tests  are  usually  measured  by  means  of  the  spark  gauge. 
This  consists  of  an  adjustable  air  gap,  which  is  changed  until  the 
electromotive  force  to  be  measured  is  just  able  to  strike  across 


96  ALTERNATING-CURRENT  MACHINERY 

in  the  form  of  a  spark.  The  electromotive  force  is  then  taken  from 
empirical  tables  such  as  Table  VIII,  Part  III,  page  192,  which  are 
based  upon  previous  measurements  of  the  electromotive  force  required 
to  strike  across  various  widths  of  gap.  In  the  spark  gauge  of  the 
General  Electric  Company,  the  spark  gap  is  between  metal  points, 
one  of  which  is  attached  to  a  micrometer  screw  whereby  the  gap 
space  may  be  adjusted  and  measured.  The  striking  distance  in 
any  spark  gauge  varies  greatly  with  the  condition  of  the  points. 
It  is,  therefore,  necessary  to  see  that  the  points  are  sharp  and  well 
polished  before  taking  measurements. 

Fig.  101  is  a  general  view  of  the  high-potential  testing  trans- 
former of  the  General  Electric  Company.  The  spark  gauge  is  shown 
mounted  on  top  of  the  instrument.  While  being  used,  the  gauge  is 
protected  by  a  cover.  The  function  of  this  cover  is  to  keep  the  ob- 
server's fingers  from  the  dangerous  high-voltage  points  of  the  gauge; 
and  the  cover,  moreover,  acts  as  a  switch  which  automatically  dis- 
connects the  gauge  from  the  high-voltage  terminals  of  the  trans- 
former when  the  cover  is  removed. 


ALTERNATING-CURRENT 
MACHINERY 


PART  II 
ALTERNATORS 

—  -  - 

Fundamental  Equation  of  Alternator.  The  equation  express- 
ing the  effective  electromotive  force  of  an  alternator  in  terms  of 
the  useful  magnetic  flux  per  pole,  the  number  of  poles,  the  number  of 
armature  conductors,  and  the  speed  of  the  armature,  is  called,  from 
its  importance  in  calculations  in  designing,  the  fundamental  equation 
of  the  alternator.  This  equation  is 

olts  (29) 


in  which  E  is  the  effective  electromotive  force  of  the  alternator; 
K  is  what  we  shall  call  the  electromotive  force  factor  of  the  machine 
and  its  value  depends  upon  the  ratio  of  breadth  of  pole  face  to  pole 
pitch,  and  upon  the  distribution  of  windings  upon  the  armature  core  ; 

p  is  the  number  of  poles  of  the  field 
magnet;  <I>  is  the  useful  magnetic  flux 
per  pole,  that  is,  the  number  of  lines  of 

^  "^  ^S^s^          magnetic  flux  that  cross  the  gap  from 

\-          ___  y>          one  pole  into  the  armature;  n  is  the 

\s  ^Ns//^  speed  of  the  armature  in  revolutions  per 

Fig.  102.    Concentrated  or  Uni-coii      second;andZis  the  total  number  of  con- 

ductors on  the  surface  of  the  armature. 

We  shall  discuss  this  equation  for  the  simplest  case  first,  that  is, 
when  the  armature  conductors  are  concentrated  in  one  slot  per  pole. 
This  type  of  winding  is  called  a  concentrated,  or  uni-coil,  winding, 
illustrated  in  part  in  Fig.  102. 

A  given  conductor  cuts  p$  lines  of  force  in  passing  all  of  the 
poles  in  one  revolution,  and  since  the  armature  makes  n  revolutions 
per  second,  the  given  conductor  cuts  np$>  lines  of  force  per  second. 
Now,  by  definition,  the  cutting  of  one  line  of  force  per  second  induces 

Copyright  1912,  by  American  School  of  Correspondence. 


98  ALTERNATING-CURRENT  MACHINERY 

in  a  conductor  one  c.  g.  s.  (centimeter-gram-second)  unit  of  electro- 
motive force.  Therefore,  there  is  an  average  of  npfy  c.  g.  s.  units 
of  electromotive  force  induced  in  one  armature  conductor;  but 
since  there  are  Z  armature  conductors  in  series  between  the  col- 
lector rings,  the  average  electromotive  force  between  collector  rings  is 

Znp3> 

Znp3>  c.  g.  s.  units,  or  -      -  volts. 
108 

The  factor  by  which  the  average  electromotive  force  must  be 
multiplied  to  give  the  effective  electromotive  force  is  called  the 
form  factor  of  the  electromotive  force  curve  of  the  alternator.  There- 
fore, if  K  is  this  form  factor,  we  have 

KZnp$> 

effective  E=  -  volts 
108 

& 

Since  —  =  T,  or  Z=  2  T  and  since  pn=  2f,  this  equation  may  be  written 

ft 

so  as  to  give  the  electromotive  force  (effective)  of  the  alternator  in 
terms  of  the  number  of  turns  T  of  wire  on  the  armature,  and  of  the 
frequency  /,  as  follows  : 

volts  (30) 


For  example,  an  alternator  has  200  turns  of  wire  on  its  arm- 
ature and  1,000,000  lines  of  magnetic  flux  from  each  field  pole. 
It  is  run  to  give  a  frequency  of  125  cycles  per  second.  The  value 
of  the  factor  K  is  1.11,  assuming  a  sine-wave  electromotive  force 
curve  and  concentrated  winding.  The  effective  electromotive  force 
of  this"  alternator,  therefore,  is 

4X  1.11  X  106X  125  X  200 

.  -  =  1,110  volts 

Electromotive  Force  Factor  K  in  Equation  (29).  When  the 
magnetic  flux  in  the  air  gap  is  distributed  in  the  ideal  way  explained 
below  and  represented  in  Fig.  103,  the  factor  K  is  called  the  phase 
constant  of  the  winding. 

When  the  winding  is  concentrated,  the  factor  K  is  called  the 
form  factor  of  the  electromotive  force  curve.  This  factor  K  depends 
in  general  upon  the  manner  in  which  the  magnetic  flux  is  distributed 


ALTERNATING-CURRENT  MACHINERY 


99 


in  the  air  gap,  and  upon  the  manner  in  which  the  armature  windings 
are  distributed  around  the  armature. 

CASE  1 .    When  a  harmonic  electromotive  force  is  induced  in  each 
turn  of  the  armature  winding.    This  is  the  case — never  fully  realized 


Fig.  103. 


Ideal  Distribution  of 
Magnetic  Flux 


Fig.   104.     Armature  Winding  Diagram, 
Four  Slots  per  Pole 


in  practice — when  the  magnetic  flux-density — that  is,  the  field 
intensity  in  the  gap  space  between  the  pole  faces  and  the  iron  of  the 
armature  core —  is  zero  at  the  points  a,  Fig.  103,  and  when  this  field 
intensity  increases  to  a  maximum  at  c  and  at  c'  in  such  a  manner 
that  the  field  intensity  at  any  point  b  is 
proportional  to  the  sine  of  the  angle  /?. 
Consider  an  armature  rotating  in  a 
magnetic  field  distributed  in  the  ideal  way 
above  specified.  Suppose  the  winding  to 
be  arranged  in  slots  spaced  as  shown  in 
Fig.  104,  four  slots  per  pole.  Fig.  105 
shows  one  group  abed  of  these  slots 
drawn  to  a  larger  scale.  Two  wires  on 
the  armature,  at  a  distance  apart  equal 
to  the  distance  between  adjacent  north 
poles,  and  subtending  the  angle  q,  Fig. 
105,  have  induced  in  them  two  electro- 
motive forces.  These  electromotive  forces 
are  to  be  thought  of  as  differing  in  phase 
by  360°,  because  of  the  fact  that  the  electromotive  force  in  a  given  con- 
ductor passes  through  a  complete  cycle  while  the  conductor  moves  from 
the  center  of  a  given  north  pole  to  the  center  of  the  next  north  pole. 


Fig.   105.     Enlarged   Section 
of_Fig.  104 


100  ALTERNATING-CURRENT  MACHINERY 

Therefore,  the  phase  difference  of  the  electromotive  forces  induced 
in  the  wires  placed  in  slot  a,  and  those  induced  in  the  wires  placed  in 

o 

slot  6,  is  -  -  X  360;  or,  in  other  words,  this  angle  is: 

width  of  tooth  -f-  width  of  slot 
circumference  of  armature  -J-  number  of  pairs  of  poles 

The  lines  A  and  B  in  the  clock  diagram,  Fig.  106,  represent  in 
magnitude  and  phase  the  electromotive  forces  induced  in  the  wires 

in  slots  a  and  b,  respectively, 
Fig.  104.  Similarly,  the  lines  C 
and  D  in  Fig.  106*  represent  the 
electromotive  forces  induced  in 
the  wires  in  slots  c  and  d,  re- 
spectively, Fig.  104.  If,  now,  the 
windings  in  the  slots  a,  b,  c,  and 

Fig.  106.    Clock  Diagram  of  Induced  E.  M.  F.'s 

for  Fig.  105  dt  Fig.   104,  are  as  in  practice 

connected  in  series,  the  total  electromotive  force  produced  by  all 
the  windings  will  be  represented  by  the  line  E  in  Fig.  106.  The 
line  E  is  the  closing  side  of  the  polygon  of  which  the  sides  A,  E',  C", 
and  D'  are  drawn  respectively,  parallel  and  equal  to  the  electromotive 
force  lines  A,  B,  C,  and  D.  The  value  of  K,  which,  in  the  case  of 
the  ideally  distributed  field  flux  here  considered,  is  called  the  phase 

F 

constant  of  the  winding,  is  equal  to  the  ratio  — ;  that  is,  the  value 

4./1 

of  K  is  equal  to  the  ratio  of  the  length  of  the  line  E  to  four  times  the 
length  of  one  of  the  lines  A,  B,  C,  or  D. 

CASE  2.  When  a  harmonic  electromotive  force  is  not  induced  in 
each  turn  of  the  armature  winding.  We  shall  discuss,  (a)  the  case  of 
a  concentrated  winding  in  which  case  K  is  simply  the  form  factor  of 
the  electromotive  force  wave;  and  (b)  the  case  of  a  distributed 
winding. 

(a)  Fig.  107  shows  a  developed  view  of  a  four-pole  alternator 
having  four  armature  conductors  a,  b,  c,  and  d,  represented  by  the 
symbols  0  and  0  depending  upon  whether  the  induced  electro- 
motive forces  are  directed  towards  or  away  from  the  reader,  respect- 
ively. Of  course,  these  four  conductors  are  connected  in  series  be- 
tween the  collecting  rings  (not  shown  in  the  figure)  and  in  tracing 


ALTERNATING-CURRENT  MACHINERY 


101 


the  circuit  from  one  collecting  ring  to  the  other,  one  would  pass  down 
along  conductor  a,  then  across  to  conductor  6,  up  b,  then  across  to 
conductor  c,  down  c,  then  across  to  conductor  d,  up  d,  and  then  to  the 
other  collecting  ring. 

Let  time  be  reckoned  from  the  instant  when  conductor  a  is  in 
the  position  shown;  and  let  time  be  plotted  as  abscissas,  and  suc- 
cessive values  of  the  induced  electromotive  force  as  ordinates,  in 
the  diagram  AB,  Fig.  107.  Suppose  that  the  intensity  of  the  magnetic 
field  in  the  gap  space  between  pole  faces  and  armature  core  is  uni- 
form, and  that  it  terminates  sharply  at  the  pole  tips,  that  is,  that  there 
is  no  spreading  of  the  lines  of  force  such  as  is  shown  in  Fig.  103; 
as  a  matter  of  fact,  however,  the  field  always  does  spread.  The  arm- 
ature conductors  move  with  uniform  velocity  to  the  right,  and  the 
ratio  of  breadth  of  pole  face,  6  inches,  to  pole  pitch,  10  inches,  is  .6. 

Then,  during  each  cycle  the  duration  of  which  is  20  units  of 
time,  the  successive  instantaneous  values  of  the  induced  electromo- 
tive force  are:  constant,  positive,  and  equal  to  E  for  6  units  of  time; 


-*-  6"-* 
N 

.£. 

- 

N 

S 

6  unite  motion 

r/me 

|  |    Ax/'s  of  time      B 


\\Qun/te  fime  \  f/'me  time 

Fig.   107.     Development  and  E.  M.  F.  Curve  for  a  Four-Pole  Alternator 

zero  for  4  units  of  time;  constant,  negative,  and  equal  to  E  for  6 
units  of  time;  and  again  zero  for  4  units  of  time.  The  average  value 
of  the  electromotive  force  during  the  first  half  cycle  is,  therefore, 
equal  to 

(EX  6)  +(0X4) 


10 


=  .6  E  volts 


The  squares  of  the  successive  instantaneous  values  of  the  in- 
duced electromotive  force  are:  constant,  positive,  and  equal  to  E2 
for  6  units  of  time;  zero  for  4  units  of  time;  and  so  on.  The  average 
value  of  the  squares  of  the  successive  instantaneous  values  of  the 
bduced  electromotive  force  during  half  a  cycle  is,  therefore,  equal  to 


102 


ALTERNATING-CURRENT  MACHINERY 


(£2X6)+(02X4) 
10 


volts2  =  .6  E2  volts2 


or 


effective  value  of  induced  electromotive  force  =  V  .6    X  E  volts 

The  value  of  K  (form  factor),  in  this  particular  case  of  a  con- 
centrated winding,  is 

effective  value  _  1/.6  E 
average  value        .6  E 


K 


1.29 


The  value  of  K  for  a  concentrated  winding  may  be  calculated,  as 
in  the  above  example,  for  any  breadth  of  pole  face. 

(b)     The  value  of  K,  in  the  case  of  a  distributed  winding,  is  cal- 


Fig.   108.     Development  and  E.  M.  F.  Curve  of  Four-Pole  Alternator  with  Two 
Conductors  for  Each  Field  Pole 

culated  as  follows,    assuming,   for  the  sake  of  clearness,   a  ratio  of 

12 
pole  breadth  to  pole  pitch  of  —  =  0.6. 

Fig.  108  shows  a  four-pole  alternator  with  two  armature  con- 
ductors a  and  b  for  each  field  pole.  (Only  two  of  these  conductors 
are  shown  in  the  figure.)  The  curve  of  the  electromotive  force 
induced  in  all  the  a  conductors  is  shown  by  the  rectangular  waves 
AB,  This  electromotive  force  has  a  constant,  positive  value  E 


rt  -«    p 


ALTERNATING-CURRENT  MACHINERY  103 


for  12  units  of  time;  then  a  zero  value  for  8  units  of  time;  then  a 
constant,  negative  value  E  for  12  units  of  time;  and  so  on. 

The  curve  of  the  electromotive  force  induced  in  all  the  b  con- 
ductors is  shown  by  the  rectangular  waves  CD.  This  electromo- 
tive force  rises  from  zero  to  the  full  value  E  at  the  instant  when 
the  conductors  b  are  in  the  position  of  a,  as  shown  in  the  figure; 
that  is,  the  electromotive  force  induced  in  conductors  b  rises  from 
zero  to  its  full  value  E,  7  units  of  time  before  the  corresponding 
rise  of  the  electromotive  force  occurs  in  conductors  a. 

The  total  electromotive  force  curve  EF  is  found  by  adding 
corresponding  ordinates  of  the  curves  AB  and  CD.  A  careful 
inspection  and  comparison  of  AB  and  CD  shows  that  the  total 
electromotive  force  of  the  alternator  is  zero  for  1  unit  of  time,  equal 
to  E  for  7  units  of  time,  equal  to  2E  for  5  units  of  time,  and  equal 
to  E  for  7  units  of  time,  during  each  half  cycle  of  20  units  of  time. 
Therefore,  the  average  value  of  the  electromotive  force  EF  during 
half  a  cycle  is 

(OX1)+(£X7)+(2£X5)+(EX7)       24 


Referring  to  the  curve  EF,  Fig.  108,  it  is  evident  that  the 
squares  of  the  successive  instantaneous  values  of  the  total  electro- 
motive force  of  the  machine  are  as  follows:  O2  for  one  unit  of  time, 
E2  for  seven  units  of  time,  (2E)2  for  five  units  of  time,  and  E2  for 
seven  units  of  time,  during  each  half  cycle  of  twenty  units  of  time. 
Therefore,  the  average  value  of  the  squares  of  the  electromotive 
force  EF  during  half  a  cycle,  is 

(02X  1)  +  (E2X  7)  +  (4E2X  5)  +  (#2X  7)  _  34 

— •  /\  Hi 

and  the  effective  value  of  the  electromotive  force  EF  is 

|ix£  =  1.304  E 
\  20 

The  value  of  K  for  the  special. case  under  consideration,  as  shown 
in  Fig.  108,  is  the  ratio 

effective  electromotive  force  _  1 .304  _ 
average  electromotive  force        1.2 


104 


ALTERNATING-CURRENT  MACHINERY 


which  is  simply  the  form  factor  of  the  electromotive  force  curve 
of  the  alternator.  The  form  factor  of  a  sine  wave  electromotive 
force  has  already  been  shown  to  be  1.11. 

NOTE. — The  factor  K  is  the  same  thing  as  form  factor  whenever  the 
distance  ab,  Fig.  108,  between  the  remotest  conductors  of  a  group  of  con- 
ductors is  less  than  the  distance  cd  between  the  pole  tips,  on  the  assumption 
that  the  magnetic  lines  of  force  do  not  spread  into  the  spaces  between  the 
pole  tips. 

Armature  Reaction.  The  current  that  circulates  in  an  alter- 
nator armature  has  magnetizing  action;  and  the  actual  useful  flux 
<I>  per  pole  is  due  to  the  combined  magnetizing  action  of  the  field 
coils  and  of  the  armature  coils.  This  magnetizing  action  of  the 
armature  current  with  respect  to  its  effect  upon  the  useful 
flux  cp,  is  called  armature  reaction.  In  case  the  current  in  the  arma- 
ture lags  behind  the  electromotive  force — when,  for  example,  the 
outside  receiving  circuit  has  inductance,  as  when  the  alternator 

supplies  current  to  induction  motors — 
the  effect  of  armature  reaction  is  to 
reduce  the  useful  flux  <£  from  each  pole. 
In  case  the  current  in  the  armature  is 
ahead  of  the  electromotive  force  in  phase 
(a  condition  that  obtains  when  the  alter- 
nator supplies  current  to  an  over-excited 
synchronous  motor  or  to  any  receiving 
apparatus  over  a  long  transmission  line, 
or,  in  general,  when  the  receiving  appa- 
ratus acts  like  a  condenser),  the  effect 
of  armature  reaction  is  to  increase  the 
useful  flux  <I>  from  each  pole. 

To  state  the  matter  in  another  way,  it  may  be  said  that  the  effect 
of  a  lagging  armature  current  is  to  oppose  the  magnetizing  action  of 
the  field  coils.  On  the  other  hand,  the  effect  of  a  leading  current 
in  the  armature  is  to  help  the  magnetizing  action  of  the  field  coils. 
In  an  alternator  the  invisible  variations  in  phase  difference 
between  the  armature  current  and  the  electromotive  force,  due  to 
the  varying  character  of  the  receiving  apparatus,  correspond,  in 
their  influence  on  armature  reaction,  to  the  visible  variations  in  the 
position  of  the  brushes  of  a  direct-current  generator. 


Fig.  109.     Portion  of  Armature 
and  Field  Coil  for  Single- 
Phase  Alternator 


ALTERNATING-CURRENT  MACHINERY  105 

Fig.  109  represents  a  single-phase  alternator  of  the  revolving 
armature  type  running  in  the  direction  indicated  by  the  curved 
arrow.  The  electromotive  force  induced  in  the  armature  coil  A 
is  zero  at  the  instant  when  the  armature  is  in  the  position  shown;  and 
if  the  armature  current  is  in  phase  with  the  induced  electromotive 
force,  the  current  also  will  be  zero  at  this  instant. 

In  considering  armature  reaction  we  shall  discuss  three  cases, 
as  follows : 

1.  Armature  current  in  phase  with  the  electromotive  force 

2.  Armature  current  lagging  behind  the  electromotive  force. 

3.  Armature  current  leading  the  electromotive  force. 

CASE  1.  As  the  armature  tooth  A,  Fig.  109,  passes  by  the 
field  pole  N,  the  current  in  the  armature  coil  on  A  is  reversed  ii. 
direction.  If  the  current  is  in  phase  with  the  electromotive  force 
induced  in  the  armature,  this  reversal  of  direction  of  current  occur: 
at  the  instant  when  the  tooth  A  is  squarely  under  N.  In  thi 
case  the  armature  current  flowing  in  coil  A  just  previous  to  the 
reversal,  that  is,  when  the  tooth  A  is  approaching  N,  opposes  by 
its  magnetizing  action  the  flux  from  N;  and  after  reversal  the  cur- 
rent in  the  coil  A  helps  the  flux  from  N.  Therefore,  the  former 
or  demagnetizing  action  of  coil  A  is  balanced  by  the  subsequent 
magnetizing  action;  and  the  only  effect  of  the  armature  current 
in  A  is  to  weaken  the  one  side  of  the  pole  N,  and  to  strengthen  to 
an  equal  extent  the  other  side,  thus  leaving  the  useful  flux  <$  un- 
changed. 

CASE  2.  When  the  current  lags  behind  the  electromotive 
force,  the  reversal  of  current  in  the  coil  A  occurs  at  a  later  instant 
than  in  Case  1,  that  is,  when  the  coil  A  has  passed  beyond  the 
center  of  the  pole  N.  Hence,  the  demagnetizing  action  of  the  cur- 
rent in  coil  A  before  reversal  lasts  for  a  longer  time  than  the  mag- 
netizing action  of  the  current  in  coil  A  after  reversal,  and  the  de- 
magnetizing action  exceeds  the  magnetizing  action.  Therefore, 
the  resultant  effect  of  the  armature  reaction  is  to  decrease  the  use- 
ful flux  <|>  from  the  pole  N. 

CASE  3.  When  the  current  is  in  advance  of  the  electromotive 
force  in  phase,  the  reversal  of  current  in  the  coil  A  occurs  at  an 
instant  earlier  than  in  Case  1,  that  is,  before  the  coil  A  has  reached 
the  center  of  the  pole  N.  Hence,  the  demagnetizing  action  of  the 


106  ALTERNATING-CURRENT  MACHINERY 

current  in  coil  A  before  reversal  lasts  for  a  shorter  time  than  the 
magnetizing  action  of  the  current  in  coil  A  after  reversal,  and  the 
magnetizing  action  exceeds  the  demagnetizing  action.  Therefore, 
the  resultant  effect  of  the  armature  reaction  is  to  increase  the  use- 
ful flux  <|>  from  the  pole  N. 

Armature  Inductance.  The  value  of  the  inductance  of  an 
alternator  armature  varies  with  the  position  of  the  armature  coils 
with  respect  to  the  field-magnet  poles,  so  that  the  inductance 
of  an  armature  increases  and  decreases  at  a  frequency  twice*  as 
great  as  the  frequency  of  the  electromotive  force  of  the  alternator. 
It  is  helpful  to  remember  that  inductance  is  proportional  to  the 
product  of  the  magnetic  flux  into  the  number  of  turns  threaded  by 
this  flux,  divided  by  the  amperes  passing  through  the  turns.  The 
armature  of  the  alternator  shown  in  Fig.  1,  page  2,  for  example, 
has  about  three  or  four  times  as  much  inductance  when  the  arma- 
ture teeth  are  squarely  under  the  field  poles  as  it  has  when  the  ar- 
mature teeth  are  midway  between  field  poles.  That  is,  the  mag- 
netic flux  produced  through  the  armature  teeth  by  a  given  current 
is  three  or  four  times  as  great  in  the  first  case  as  in  the  second  case. 
This  fluctuation  of  armature  inductance  makes  it  very  difficult 
to  predetermine  the  electromotive  force  and,  in  general,  the  be- 
havior of  a  machine.  In  the  following  discussion  the  armature 
inductance  is  assumed  to  be  constant. 

The  inductance  of  an  alternator  armature  is  proportional  to 
the  linear  dimensions  of  the  armature,  other  things  being  equal; 
and  the  inductance  of  an  armature  of  given  size  and  given  total 
number  of  turns  is  much  greater  when  the  winding  is  concentrated 
than  it  is  when  the  winding  is  distributed. 

A  moderate  amount  of  armature  inductance  is  advantageous 
in  alternators  which  are  to  be  run  in  parallel;  and  in  case  of  a  short- 
circuit,  the  armature  inductance  keeps  the  current  from  becoming 
excessive.  On  the  other  hand,  armature  inductance  is  more  or  less  ob- 
jectionable in  an  alternator  which  is  to  be  used  to  supply  current  at 
constant  electromotive  force,  on  account  of  the  electromotive  force 
that  is  lost  in  the  armature. 


*The  electromotive  forde  of  an  alternator  passes  through  a  cycle  when  an  armature 
coil  passes  from  a  north  pole  of  the  field  to  the  next  north  pole.  The  inductance  passes 
through  a  cycle  of  values  when  an  armature  coil  passes  from  one  field  pole  to  the  next  field 
pole. 


ALTERNATING-CURRENT  MACHINERY  107 

The  inductance  of  an  armature  is  best  determined  by  sending 
through  it  when  at  rest,  from  an  outside  source,  a  measured  alter- 
nating current  7,  and  measuring  the  electromotive  force  E  (volts 
drop)  between  the  collecting  rings.  Then 


or,  solving  for  L,  we  have 

1 

L  =  —  l/E2  -  (IR)*  (31) 

Jco 

Knowing  the  armature  resistance  and  the  frequency  — ,  we   can 

find  L  from  equation  (31).  The  value  of  L  thus  calculated  depends 
greatly  upon  the  position  in  which  the  armature  is  held,  as  explained 
above,  and  also  upon  the  degree  of  field  excitation. 

For  example,  the  armature  of  a  certain  single-phase  alternator 
has  a  resistance  of  0.2  ohm  measured  between  collector  rings.  An 
electromotive  force  of  100  volts  at  a  frequency  of  125  cycles  per 
second  (&>=785  radians  per  second)  applied  to  the  collecting  rings 
of  the  armature  at  rest,  produces  an  effective  current  of  100  am- 
peres. Therefore,  the  inductance  of  the  armature,  as  calculated  by 
use  of  equation  (31),  is 

L=         l 


100785 

Electromotive  Force  Lost  in  Armature  Drop.  The  electro- 
motive force  between  the  collecting  rings  of  an  alternator  with 
given  load,  is  less  than  the  electromotive  force  between  rings  at 
zero  load,  with  given  field  excitation,  because  of  two  electromo- 
tive force  losses  that  occur,  and  because  of  the  effect  of  armature 
reaction. 

(a)  The  loss  of  electromotive  force,  or  the  drop,  is  due,  in  the  first 
place,  to  the  resistance  of  the  armature.     This  loss  is  equal  to  IR;  it  is  in 
phase  with  7;  and  it  is  precisely  analogous   to  the  electromotive  force  lost 
in  a  direct-current  armature  due  to  the  resistance  of  the  armature.     This 
IR  drop  is  of  relatively  small  value  and  importance. 

(b)  The  loss  of  electromotive  force,  or  the  drop,  is  due,  secondly,  to 
the  inductance  of  the  armature.     This  loss  is  equal  to  ajLI,  and  it  is  90  degrees 
ahead  of  7  in  phase. 


108  ALTERNATING-CURRENT  MACHINERY 

(c)  The  demagnetizing  action  of  the  armature  current  on  the  field 
lessens  the  useful  flux,  and  thus  indirectly  causes  a  falling-off  in  the  induced 
electromotive  force. 

The  result  of  the  actions  (b)  and  (c)  above  is  to  cause  a  loss  of 
electromotive  force  in  the  armature  of  the  same  character  in  each 
case  in  so  far  as  phase  relations  with  current  are  concerned.  There- 
fore, it  is  convenient  to  attribute  the  total  effect  of  (b)  and  (c)  to 
a  fictitious  armature  inductance  Lf,  which  is,  of  course,  larger  in 
value  than  the  armature  inductance  L  in  equation  (31).  The 
inductance  reactance  coLf  corresponding  to  this  equivalent  induct- 
ance L',  is  called  the  synchronous  reactance  of  the  armature. 

Alternator  Regulation.  Given  an  alternator,  having  constant 
field  excitation.  It  has  a  certain  electromotive  force  between  col- 
lecting rings  when  its  current  output  is  zero.  As  the  current  out- 
put increases,  the  electromotive  force  between  collector  rings  gen- 
erally decreases,  because  of  the  actions  already  described;  and, 
conversely,  as  the  current  output  decreases,  the  terminal  electro- 
motive force  rises.  The  increase  of  electromotive  force  from  full 
load  to  zero  load,  with  constant  full-load  field  excitation  and  con- 
stant speed  of  driving,  expressed  as  a  percentage  of  the  full-load 
terminal  electromotive  force,  is  called  regulation  of  the  alternator. 

For  example,  a  certain  alternator  gives  1,100  volts  between  its 
collector  rings  at  full-load  current  and  full-load  field  excitation. 
When  the  current  output  is  decreased  to  zero  by  opening  the  main 
switch,  leaving  the  field  excitation  and  speed  unchanged,  the  ter- 
minal electromotive  force  rises  to  1,166  volts.  The  regulation  is, 
therefore, 

1,166-  1,100 

1,100         X  100  =6  per  cent 

The  regulation  of  a  given  alternator  varies  greatly  with  the 
character  of  the  receiving  circuit  to  which  it  delivers  current. 
When  the  receiving  circuit  has  large  inductance  reactance  (as  in 
the  case  of  under-loaded  transformers  and  induction  motors),  the 
terminal  electromotive  force,  under  increasing  load,  falls  off  very 
much  more  than  when  the  receiving  circuit  is  non-inductive  (as,' 
for  example,  when  the  receiving  circuit  consists  of  incandescent 
lamps  supplied  through  fully  loaded  transformers).  In  other 
words,  the  regulation  of  an  alternator  is  larger  (i.  e.,  poorer)  for  in- 


ALTERNATING-CURRENT  MACHINERY  109 

ductive  receiving  circuits  than  for  non-inductive  receiving  circuits. 
If  the  receiving  circuit  has  large  capacity  reactance  (as,  for  example, 
when  the  receiving  circuit  consists  of  over-excited  synchronous 
motors),  the  terminal  electromotive  force  of  the  alternator  will  rise 
with  an  increase  of  the  current  output;  and  the  regulation  of  the 
alternator  will  be  negative.  In  practice,  the  receiving  circuit  never 
as  a  whole  has  capacity  reactance. 

For  example,  a  given  alternator  having  a  regulation  of  8  per 
cent  on  a  non-inductive  receiving  circuit  (unity  power  factor),  has 
a  regulation  of  about  21  per  cent  on  an  inductive  receiving  circuit 
having  a  power  factor  of  0.9  and  a  regulation  of  about  26  per  cent 
at  a  power  factor  of  0.8  (lagging). 

Field  Excitation.  In  most  alternating-current  systems  the 
voltage  at  the  points  from  which  current  is  distributed  is  kept  con- 
stant or  approximately  constant.  This  requires  that  the  voltage 
at  the  terminals  of  the  alternator  be  somewhat  increased  as  the 
amount  of  current  (or  load)  is  increased,  the  amount  of  the  increase 
in  electromotive  force  depending  on  the  volts  lost  in  the  line.  If 
the  field  excitation  of  an  alternator  be  kept  constant  while  the 
current  taken  from  the  armature  is  increased,  the  voltage  at  the 
terminals  will  decrease,  just  as  in  the  case  of  a  direct-current  shunt 
generator.  Hence,  in  order  to  keep  the  voltage  at  the  terminals 
constant,  or  to  cause  a  rise  of  voltage  with  increasing  current  output, 
it  becomes  necessary  to  increase  the  field  excitation  with  increasing 
current  output. 

There  are  in  general  three  methods  in  use  for  accomplishing 
this  voltage  control. 

(1)     By  varying  the  field  excitation  with  the  load. 

(a)  Through  control  of  the  field-exciting  current  of  the  alter- 
nator by  a  rheostat  operated  either  by  hand  or  automatically  from 
the  alternator. 

(b)  Through  control  of  the  exciter  itself  by  the  main  current 
from  the  alternator  with  or  without  a  rheostat.     This  is  accomplished 
in  one  of  the  three  following  ways:  first,  compounding  the  exciter 
with  rectified  current  supplied  to  its  field  circuit;   second,  by  sup- 
plying the  armature  circuit  of  the  exciter  with  alternating  current 
from  the  alternator   (compensated  field  method);  and  third,  by  an 
external  regulator  for  varying  the  exciter  field  current  by  rapidly 


110 


ALTERNATING-CURRENT  MACHINERY 


short-circuiting  its  field  rheostat,  the  duration  of  the  periods  of 
short-circuit  depending  on  the  terminal  voltage  of  the  alternator 
(Tirrill  regulator). 

(2)  By  interaction   between  the  fields  of  the   alternator  and 
its  exciter.     This  is  Heyland's  method  which  is  used  abroad  but 
not  in  this  country.     Use  is  made  of  the  stray  flux  from  the  alter- 
nator field  which  is  arranged  to  strengthen  the  field  of  the  exciter 
and  thus  obtain  a  compounding  effect.     The  reaction  on  the  field 
of  the  exciter  is  proportional  to  the  armature  reaction  of  the  alter- 
nator and  the  terminal  voltage  of  the  exciter  follows  closely  the 
variations  in  the  load  on  the  alternator. 

(3)  By  utilizing  the  magnetic  flux  due  to  the  armature  cur- 
rent so  that  the  armature  reaction  of  the  alternator  increases  the 

total  flux  per  pole,  and  thus 
increases  the  voltage  of  the 
alternator  as  the  load  in- 
creases. The  exciting  field 
current  itself  is  not  varied. 
This  is  Walker's  method  and 
it  has  been  used  by  the  Brit- 
ish Westinghouse  Company. 
Of  the  above  methods 
the  first  includes  practically 
all  that  are  used  at  present 
in  this  country.  The  tend- 
ency is  to  abandon  the 

attempts  to  design  alternators  to  be  inherently  self -regulating  and 
to  avoid  as  far  as  possible  all  special  devices  internal  to  the  alterna- 
tor and  its  exciter  for  securing  automatic  voltage  control,  and  to 
adopt  instead  an  automatic  regulator  external  to  the  alternator. 

Under  method  (1)  will  be  described  the  three  commonest  ways 
of  voltage  control  employed  at  present,  namely,  separate  excitation; 
composite  excitation,  and  the  automatic  regulator,  external  to  the 
alternator. 

CASE!.  Separate  excitation.  The  simplest  method  is  that  illus- 
trated by  the  diagram,  Fig.  110,  in  which  A  represents  the  armature 
winding,  the  terminals  of  which  T\  and  T<2  are  connected  to  the  collector 
rings,  which  in  turn  are  connected  to  the  line  wires  through  the  brushes. 


Fig.   110.     Diagram  Showing  Method  of  Separate 
Excitation 


ALTERNATING-CURRENT  MACHINERY 


111 


The  field  of  the  alternator  is  excited  by  a  set  of  coils  on  the 
pole  pieces.  These  coils  are  represented  by  F;  and  current  is  sup- 
plied to  these  coils  from  a  small  direct-current  dynamo  E,  called 
the  exciter.  This  exciter  is  a  small  direct-current  shunt-wound  or 
compound-wound  dynamo  furnished  with  an  adjustable  rheostat  r 
in  series  with  its  field  /.  An  adjustable  rheostat  R  is  placed  in  the 
alternator  field  circuit  also.  When  the  electromotive  force  of  the 
alternator  decreases,  its  field  may  be  strengthened  by  cutting  out 
resistance  in  either  R  or  r,  or  in  both. 

Regulation  by  r  alone  is  generally  used  in  large  machines, 
since  the  exciter's  field  current  is  relatively  small,  while  the  alter- 
nator field  current  is  usually 
large  and  hence  would  cause  a 
large  I2  R  loss  if  passed  through 
a  rheostat.  Separate  excitation 
is  still  used  in  some  of  the  older 
electric  lighting  stations. 

CASE  2.  Composite  excita- 
tion. The  electromotive  force  of 
an  alternator  excited  as  in  Case 
1  falls  off  greatly  with  increasing 
current  output ;  and  to  counteract 
this  tendency  automatically,  an 
auxiliary  field  excitation  is  some- 
times provided,  which  increases 
with  the  current  output  of  the 
machine.  For  this  purpose  the 
whole  or  a  portion  of  the  current 
given  out  by  the  machine  is  recti- 
fied,* and  sent  through  the  auxiliary  field  coils.  This  arrangement 
is  shown  in  Fig.  111. 

The  field  winding  of  the  alternator  has  two  sets  of  coils  F  and 
C.  The  coils  F  are  separately  excited  as  in  Case  1.  The  coils  C, 
known  as  the  "series"  or  "composite  coils,"  are  excited  by  the 
main  current  from  the  alternator. .  One  terminal  of  the  armature 
winding  is  connected  directly  to  a  collecting  ring.  The  other  arma- 
ture terminal  connects  to  one  set  of  alternate  bars  of  the  rectifying 

*Connections  to  field  coils  are  reversed  with  every  reversal  of  main  current,  so  that, 
in  the  field  coils,  the  current  is  uni-directional. 


Fig.   111.     Diagram  Showing  Method  of. 
Composite  Excitation 


112 


ALTERNATING-CURRENT  MACHINERY 


commutator  B.  From  the  rectifier  the  current  is  led  through  the 
winding  C,  thence  back  to  the  rectifier,  and  thence  to  the  other 
collecting  ring.  The  shunt  S,  within  the  commutator,  may  be 
used  when  it  is  desired  to  rectify  only  a  part  of  the  current.  There 
is  also  a  shunt  S'  which  may  be  used  to  regulate  the  amount  of 
current  flowing  through  the  coil  C. 

The  alternating-current  rectifier  is  an  arrangement  for  revers- 
ing the  connections  of  the  field  circuit  with  each  reversal  of  the 
current  from  the  alternator,  so  that  the  current  may  flow  always 
in  the  same  direction  in  the  field  circuit.  The  rectifier  is  a  commu- 
tator mounted  on  the  armature  ^shaft.  This  commutator  has  as 

many  bars  as  there  are  poles  on 
the  field  magnet  of  the  alternator. 
These  bars  are  wide,  and  are 
separated  by  quite  narrow  spaces 
filled  with  mica  insulation.  Let 
these  bars  be  numbered  in  order 
around  the  commutator.  The 
even-numbered  bars  are  connect- 
ed together,  and  the  odd-number- 
ed bars  are  connected  together. 
The  connecting  wire  leading  from 
one  terminal  of  the  alternator 
armature  to  one  of  the  collector 
rings  is  cut ;  and  the  two  ends  thus 
formed  are  connected,  one  to  the 
even-numbered  bars  (shown  in 
full  black  in  Fig.  Ill)  of  the  rec- 
tifying commutator,  and  the  other  to  the  odd-numbered  bars  (shown 
white  in  Fig.  111).  The  field  circuit  that  is  to  receive  the  rectified 
current  is  connected  to  two  brushes  which  rub  on  the  rectifying 
commutator,  these  brushes  being  so  spaced  that  one  touches  an 
odd-numbered  bar  when  the  other  touches  an  even-numbered  bar. 
The  brushes  are  carried  in  a  rocker  arm,  which  is  moved  forwards 
or  backwards  until  the  brushes  are  passing  from  one  bar  to  the 
next  at  the  instant  that  the  alternating  current  from  the  alternator 
is  passing  through  the  value  zero.  The  proper  adjustment  of  the 
brushes  is  indicated  by  a  minimum  of  sparking. 


Fig.   112.     Diagram  Showing  Composite! 
Excitation  with  Transformer 


ALTERNATING-CURRENT  MACHINERY 


113 


Fig.  112  shows  an  alternator  A  with  two  sets  of  field  coils  F 
and  C  as  before.     One  armature  terminal  is  connected  to  a  collect- 

Au*///ary  Fie/cl 

fMMXlOQQOQOCXHHL 


•-200O— 
20JOO- 

KOQ 
1400 


Cornmuta  tor 


^ Q pop QQ  Series  Transformer 

Armature 


A2B2 


Fig.   113.     Two-Phase  Alternator  Diagram  with  Composite  Field 
Excitation  with  Balanced  Receiving  Circuits 

ing  ring;  and  the  other  armature  terminal  connects  to  the  primary 
of  a  transformer  T,  and  thence  to  the  other  collecting  ring.  The 
terminals  of  the  secondary  coil  of  T  connect  to  the  bars  of  the  recti- 
fying commutator  B,  from  which  the  composite  field  winding  C  is 
supplied.  The  transformer  T  is  usually  placed  inside  the  armature. 


F/e/d 


20 


OQQQQOQQ 


Corn  mut  01  tor 


Series  Transformer 
Armature 


Q&flfi2£Q 


Co//ecAor 

/T*//7< 


ABC 


Fig.  114.     Three-Phase  Alternator  Diagram  with  Composite 
Field  Excitation  with  B.alanced  Receiving  Circuits 

Composite  field  excitation  is,  however,  not  satisfactory  in  case 
'of  polyphase  alternators,  unless  the  receiving  circuits  supplied  from  the 
alternators  are  approximately  balanced.  Unbalancing  of  the  receiv- 


114  ALTERNATING-CURRENT  MACHINERY 

ing  circuits  changes  the  electromotive  forces  generated  in  the  dif- 
ferent phase  windings  of  the  armature,  by  different  amounts;  and 
composite  excitation,  applied  to  the  magnetic  field  as  a  whole, 
cannot,  properly,  correct  the  different  electromotive  force  variations 
of  the  several  phases. 

In  cases  where  the  receiver  circuits  are  approximately  balanced, 
the  current  for  the  composite  field  excitation  is  taken  through  a 
rectifying  commutator  from  the  secondary  coil  of  a  series  (or  current) 
transformer  which  has  two  or  three  distinct  primary  coils,  one  for 
each  phase.  This  arrangement  applied  to  a  two-phase  alternator 
is  shown  in  Fig.  113,  and  applied  to  a  three-phase  alternator  in 
Fig.  1 14.  The  effect  of  the  several  primary  coils  on  the  series  trans- 
former is  to  balance  up  the  slight  differences  of  the  several  poly- 
phase currents,  in  so  far  as  their  action  upon  the  composite  excita- 
tion is  concerned.  This  method  has  been  used  by  the  Westinghouse 
Company  in  the  case  of  alternators  of  small  capacity. 

CASE  3.  Automatic  regulator.  There  are  on  the  market  a 
number  of  automatic  devices  which  are  designed  to  change  the  field 
strength  of  a  generator  in  accordance  with  a  change  in  generator 
voltage.  The  most  successful  of  these  devices  is  the  Tirrill  regulator 
manufactured  by  the  General  Electric  Company.  This  regulator 
differs  from  other  types  of  regulators  in  that  it  does  not  make  use 
o?  the  principle  of  switching  resistances  in  and  out  of  the  field  circuit 
by  the  step-by-step  method.  The  Tirrill  regulator  controls  the  gen- 
erator voltage  by  rapidly  opening  and  closing  a  shunt  circuit  con- 
nected across  the  exciter  field  rheostat,  the  duration  of  such  periods 
of  short-circuit  being  varied  automatically.  The  field  rheostat  is 
first  turned  in  until  the  exciter  voltage  is  much  reduced  and  the  regu- 
lator circuit  is  then  closed.  This  short-circuits  the  rheostat  through 
contacts  in  the  regulator,  causing  the  voltage  of  the  exciter  and  the 
generator  to  immediately  increase.  At  a  predetermined  point  the 
regulator  contacts  are  automatically  opened  which  causes  the  field 
current  of  the  exciter  to  again  pass  through  the  rheostat.  The 
resulting  decrease  in  voltage  is  quickly  checked  by  another  closing 
of  the  regulator  contacts,  which  continue  to  vibrate  to  and  fro 
thus  keeping  the  generator  voltage  within  the  desired  limits. 

Fig.  115  is  a  front  view  of  a  Tirrill  regulator  "form  A2"  designed 
for  alternators  having  exciters  of  small  capacities.  Fig.  116  is  a  rear 


ALTERNATING-CURRENT  MACHINERY 


115 


Fig.   115.     Front  View  of  Tirrill 
Regulator 


view  of  the  regulator  showing  the  resistance  box  and  iron  brackets 
for  mounting  it  at  the  end  of  the 
switchboard  if  desired,  although  it  is 
recommended  that  it  be  mounted  di- 
rectly on  the  switchboard.  A  diagram 
of  the  electrical  connections  for  a  Tirrill 
regulator  with  an  alternator  and  its 
exciter  is  shown  in  Fig.  117. 

The  regulator  has  a  direct-current 
control  magnet,  an  alternating-current 
control  magnet,  and  a  relay.  The 
direct-current  control  magnet  is  con- 
nected to  the  exciter  bus  bars.  This 
magnet  has  a  fixed  stop-core  in  the 
bottom  and  a  movable  core  in  the  top 
which  is  attached  to  a  pivoted  lever 
having  at  the  opposite  end  a  flexible 

contact  pulled  downward  by  four  spiral  springs.  For  clear- 
ness, however,  only  one  spring  is  shown  in  the  diagram.  Opposite 
the  direct-current  control  magnet  is  the  alternating-current  control 
magnet  which  has  a  potential 
winding  connected  by  means  of  a 
potential  transformer  to  the  al- 
ternating-current generator  or 
bus  bars.  There  is  an  adjustable 
compensating  winding  on  the 
alternating-current  magnet  con- 
nected through  a  current  trans- 
former to  the  principal  lighting 
feeder.  The  object  of  this  wind- 
ing is  to  raise  the  voltage  of  the  al- 
ternating-current bus  bars  as  the 
load  increases.  The  alternating- 
current  control  magnet  has  a 
movable  core  and  a  lever  and  con- 
tacts similar  to  those  of  the  direct-  Fig.  116.  Back  View  of  Tirrill 

Regulator 

current  control  magnet,  and  the 

two  combined  produce  what  is  known  as  the  "floating  main  contacts." 


116 


ALTERNATING-CURRENT  MACHINERY 


The  relay  consists  of  a  U-shaped  magnet  core  having  a  differ- 
ential winding  and  a  pivoted  armature  controlling  the  contacts 
which  open  and  close  the  shunt  circuit  across  the  exciter  field  rheostat. 
One  of  the  differential  windings  of  the  relay  is  permanently  con- 
nected across  the  exciter  bus  bars  and  tends  to  keep  the  contacts 
open.  The  other  winding  is  connected  to  the  exciter  bus  bars  through 
the  floating  main  contacts  and  when  the  latter  are  closed  neutralizes 
the  effect  of  the  first  winding  and  allows  the  relay  contacts  to  short- 
circuit  the  exciter  field  rheostat.  Condensers  are  connected  across 
the  relay  contacts  to  prevent  severe  arcing  and  possible  injury. 

The  cycle  of  operation  is  as  follows:  The  circuit  shunting  the 
exciter  field  rheostat  through  the  relay  contacts  is  opened  by  means 
of  a  single-pole  switch  at  the  bottom  of  the  regulator  panel  and  the 


GSrtERATQR 

Fig.  117.     Diagram  of  Electrical  Connections  for  Tin-ill  Regulator 

rheostat  turned  in  until  the  alternating-current  voltage  is  reduced 
65  per  cent  below  normal,  which  so  weakens  both  of  the  control 
magnets  that  the  floating  main  contacts  are  closed.  This  closes  the 
relay  circuit  and  demagnetizes  the  relay  magnet,  releasing  the 
relay  armature,  and  the  spring  closes  the  relay  contacts.  The  single- 
pole  switch  is  then  closed  and  as  the  exciter  field  rheostat  is  short- 
circuited,  the  exciter  voltage  will  at  once  rise  and  bring  up  the  voltage 
of  the  alternator.  This  will  strengthen  the  alternating-current  and 
direct-current  control  magnets,  and  at  the  voltage  for  which  the 
counterweight  has  been  previously  adjusted,  the  main  contacts  will 
open.  The  relay  magnet  will  then  attract  its  armature  and  by 
opening  the  shunt  circuit  at  the  relay  contacts  will  throw  the  full 
resistance  into  the  exciter  field  circuit  tending  to  lower  the  exciter 


ALTERNATING-CURRENT  MACHINERY  117 

and  the  alternator  voltage.  The  main  contacts  will  then  be  again 
closed,  the  exciter  field  rheostat  short-circuited  through  the  relay 
contacts,  and  the  cycle  repeated.  This  operation  is  continued  at  a 
high  rate  of  vibration  due  to  the  sensitiveness  of  the  control  magnets 
and  maintains  not  a  constant  but  a  steady  exciter  voltage. 

One  of  the  advantages  of  this  regulator  is  that  in  controlling 
the  voltage  of  the  alternator  by  operating  entirely  on  the  field  cir- 
cuit of  the  exciter,  the  heating  losses  are  far  smaller  and  the  efficiency 
correspondingly  higher  than  is  the  case  with  those  regulators  which 
operate  directly  on  the  alternator  field. 

Another  advantage  is  that  several  alternators  may  be  operated 
in  parallel  using  but  one  regulator,  if  all  use  the  same  exciter.  On 
the  other  hand  if,  as  is  more  usual,  several  exciters  are  used  in 
parallel,  one  regulator  and  an  equalizer  rheostat  for  each  additional 
exciter  are  necessary. 

If  two  or  more  exciters,  not  operating  in  parallel,  are  used,  a 
separate  regulator  must  be  installed  for  each  exciter.  The  Tirrill 
"form  F"  regulator  is  made  for  large  installations,  and  is  furnished 
with  several  relays  varying  from  two  to  twelve,  according  to  the 
size,  the  capacity,  and  the  characteristics  of  the  exciters  used.  While 
these  "form  F"  regulators  differ  more  or  less  in  detail  according 
to  special  conditions,  the  main  features  of  operation  are  the  same  as 
in  the  "form  A2"  regulator. 

The  standard  voltage  for  exciters  is  125  volts,  and  in  some 
cases  as  high  as  250  volts.  Tirrill  regulators  are  designed  for  a  range 
of  from  70  to  140  volts  in  the  first  case,  and  for  a  range  of  from  140 
to  280  volts  in  the  second  case. 

With  the  growing  use  of  these  automatic  regulators  external 
to  the  alternator,  it  has  been  found  desirable  by  manufacturers 
to  minimize  their  efforts  to  design  alternators  of  low  inherent  regula- 
tion, especially  in  the  case  of  turbo-alternators  and  machines  of 
large  rated  output.  A  low  inherent  regulation  is  today  considered 
an  expensive  and  unnecessary  luxury,  for  by  some  sacrifice  in  this 
quality  a  relatively  large  gain  in  rated  capacity  becomes  possible, 
and  in  many  cases  a  higher  efficiency. 

The  advent  of  the  automatic  regulator  has  thus  enabled  de- 
signers to  effect  considerable  improvement  in  alternators  by  relieving 
them  of  the  troublesome  question  of  low  inherent  regulation,  and 


118  ALTERNATING-CURRENT  MACHINERY 

permitting  them  to  give  greater  weight  than  ever  to  the  important 
matters  of  increasing  output  and  efficiency. 

POLYPHASE  ALTERNATORS  AND  SYSTEMS 
SINGLE=PHASE  SYSTEM 

Limitations.  As  long  as  alternating  current  was  generated, 
transmitted,  and  used  for  electric  lighting  only,  the  single-phase 
system  gave  complete  satisfaction,  simplicity  in  the  generating, 
transmitting,  and  receiving  apparatus  being  its  most  striking  and 
valuable  feature. 

In  the  earlier  days  of  the  electric  lighting  industry,  there  was 
very  little,  if  any,  demand  for  current  to  operate  motors  for  power 
purposes.  Since  that  time,  however,  there  has  developed  an  ever- 
increasing  demand  for  current  for  power  purposes,  fully  equaling,  if 
not  exceeding,  that  for  lighting  work.  With  the  advent  of  this  new 
condition,  the  great  obstacle  to  the  use  of  the  single-phase  alternat- 
ing-current system  became  manifest.  Single-phase  constant-speed 
motors  are  difficult  to  make  self-starting*  under  load,  especially  in 
units  of  large  size;  and  hence  the  use  of  the  single-phase  system  for 
general  power  purposes,  with  the  apparatus  now  available,  is  not 
practicable. 

It  was  in  1888,  in  Italy,  that  Ferarris  discovered  the  important 
principle  of  the  production  of  a  rotating  magnetic  field  by  means 
of  two  or  more  alternating  currents  displaced  in  phase  from  one 
another,  and  he  thus  made  possible,  by  means  of  the  induction  motor, 
the  use  of  polyphase  currents  for  power  purposes.  The  most  important 
advantage  of  polyphase  alternating  currents  over  the  simple  single-phase 
system  is  that  alternating-current  motors  can  be  satisfactorily  operated 
by  them.  It  was  mainly  the  requirements  of  the  induction  motor 
that  led  to  the  development  of  the  polyphase  system, 

TWO=PHASE  SYSTEM 

Two-Phase  Alternator.  The  simplest  form  of  polyphase  gen- 
erator consists  of  two  similar  and  independent  single-phase  armatures 
mounted  rigidly  on  one  and  the  same  shaft,  one  beside  the  other, 
in  such  a  manner  that  the  electromotive  forces  at  the  terminals  of 


*This    statement  does  not  include  the  single-phase  series  commutator  motor  which  is 
especially  adapted  to  railway  motors. 


ALTERNATING-CURRENT  MACHINERY 


119 


the  respective  armatures  arrive  at  their  maximum  values  90  degrees, 
or  one-fourth  of  a  period,  apart.  The  currents  from  such  a  machine 
are  said  to  have  a  two-phase  relationship.  The  two  separate  armatures 
are  supposed  to  revolve  inside  the  same  crown  of  field  magnet  poles. 

Fig.  118  shows  an  end  view  of  such  an  arrangement,  but  arma- 
ture B  is  here  shown  inside  of  armature  A  for  the  sake  of  clearness. 
As  will  be  seen  in  the  figure,  armatures  A  and  B  are  so  mounted  on 
the  shaft  that  the  slots  of  A  are  midway  under  the  poles  N,  S  when 
the  slots  of  B  are  midway  between  the  same  poles.  With  this  arrange- 
ment, the  electromotive  force  generated  in  the  armature  coils  of 
A  and  B  are  so  related  in  their  variations  that  the  electromotive 
force  of  A  is  at  its  maximum  when  the  electromotive  force  of  B 
is  zero.  Or  in  other  words,  the  two  electromotive  forces  are  90 
degrees  apart  in  phase,  or  are  in 
quadrature  (at  right-angles)  to  each 
other. 

A  careful  study  of  Fig.  118 
will  show  that  the  electromotive 
forces  induced  in  armatures  A  and 
B  are  90  degrees  apart  in  phase. 
Thus  the  figure  shows  the  arma- 
ture A  in  the  position  in  which  its 
windings  (in  the  slots)  are  cutting 
lines  of  magnetic  flux  from  the 
field  poles  at  a  maximum  rate,  while 
the  armature  B  is  shown  in  the 
position  in  which  its  windings  are  midway  between  the  field  poles 
where  they  do  not  cut  any  magnetic  flux  at  all.  Therefore,  the  electro- 
motive force  in  the  windings  of  the  armature  A  is  at  its  maximum 
value,  while  the  electromotive  force  in  the  windings  of  armature  B 
is  zero  at  the  same  instant.  That  these  electromotive  forces  generated 
in  the  windings  of  A  and  B  differ  in  phase  by  90  degrees,  may  also 
be  shown  as  follows:  The  electromotive  force  generated  by  a  con- 
ductor on  armature  A  passes  through  a  complete  cycle  of  changes  as 
it  moves  from  the  center  of  one  north  pole  to  the  center  of  the  next 
north  pole.  The  interval  between  the  centers  of  two  adjacent  north 
poles,  a  "double  pole  pitch,"  corresponds  then  to  360  "electrical"  de- 
grees. Therefore,  the  phase  (space)  difference  between  the  conductors 


Fig.  118.       Diagram  of  Two-Phase  Alter- 
nator with  Two  Separate  Armatures 


120 


ALTERNATING-CURRENT  MACHINERY 


on  A  and  B  is  seen  to  be  J  of  360  degrees,  or  90  electrical  degrees. 

The  two  equal,   but  distinct  and  independent  electromotive 

forces    generated   by  such   a   two-phase    alternator   are    generally 

used  to  supply  two  distinct 
and  separate  currents  to  two 
distinct  and  independent  cir- 
cuits. When  so  used  the 
system  is  called  a  two-phase, 
four-wire  system.  We  shall  see 
later  that  it  is  possible  to  in- 
terconnect the  two  circuits 
in  such  a  manner  that  one  of 
the  four  line  wires  may  be 
omitted. 

In  practice  the  actual  two- 
phase  alternator  is  constructed 
F&slparSeaKa!±fe^hs±oCnrC±-    by  placing  both  the  armature 

windings  of  A  and  B  upon  one 

and  the  same  armature  core,  instead  of  on  separate  cores.  To  accom- 
plish this  the  armature  core  has  twice  as  many  slots  as  either  A  or  B  in 
Fig.  118.  Fig.  119  shows  such  an  armature.  The  slots  marked  a\,  a2, 
a3,  etc.,  contain  the  conductors  comprising  phase  A;  whereas  the  slots 
marked  61,  62,  63,  etc.,  contain  the  conductors  comprising  phase  B. 
The  A  winding  passes  in  slot  ai  from  front  to  back  of  the  armature 

core;  then  towards  the 
reader  (that  is,  from  back 
to  front)  in  slot  a2;  then 
from  front  to  back  in  a3, 
from  back  to  front  in  a4, 
and  so  on.  The  various 
conductors  in  slots  a\,  a2, 
a3,  etc.,  are  joined  in  series 
by  connectors  (at  front  and 
back),  and  the  two  ends  of 
the  final  series  are  connected  to  two  collector  rings. 

The  B  winding  passes  in  slot  61,  from  front  to  back  of  the  arma- 
ture core;  then  towards  the  reader,  that  is,  from  back  to  front,  in 
slot  62;  then  from  front  to  back  in  63.  from  back  to  front  in  64,  and 


Fig.  120.     Distributed  or  Multi-Coil  Winding 
for  Two-Phase  Alternator 


ALTERNATING-CURRENT  MACHINERY 


121 


ring  \ 


main  \ 


so  on.  The  various  conductors  in  slots  &i,  62,  63,  etc.,  are  joined  in 
series,  and  the  two  ends  of  the  final  series  are  connected  to  two 
collector  rings,  which  rings  are  distinct  from  the  pair  of  rings  to 
which  the  A  winding  is  connected. 
The  armature  windings  A 
and  B  just  described  are  of  the 
concentrated  or  uni-coil  type,  page 
100,  having  only  one  slot  per  pole 
for  each  winding,  i.  e.,  per  phase. 
Distributed  or  multi-coil  windings 


main   3 


Fig.  121.     Diagram  of  Collector  Ring  System 
for  Two-Phase  Alternator 


also  are  frequently  used  for  two- 
phase  alternators.  Thus,  Fig.  120 
shows  an  end  view  of  a  portion  of  a  two-phase  armature  with  its  A 
and  B  windings  each  distributed  in  two  slots  per  pole.  The  coils 
belonging  to  winding  A  are  lightly  shaded,  and  those  belonging  to 
winding  B  are  darkly  shaded  in  the  figure.  The  connections  between 
the  coils  of  the  A  winding  are  shown  in  the  figure  by  the  full  lines, 
while  the  connections  of  the  B  winding  are  shown  by  the  dotted  lines. 
Two-phase  alternators  are  usually  provided  with  two  sets  of 
collector  rings;  one  ring,  however,  may  be  made  to  serve  as  a  com- 
mon connection  for  the  two  armature  windings,  as  shown  in  Fig. 
121.  The  lines  A  and  B  in  the  clock  diagram,  Fig.  122,  represent 
the  generator  electromotive  forces,  a  represents  the  current  in 
main  l,b  represents  the  current  in  main  2,  and  cf  which  is  the  vector 
sum  of  a  and  b}  represents  the 
current  in  the  common  main  3.  If 
a  =  6,  it  is  evident  that  c  = 


Fig.   122.     Clock  Diagram  of  E.  M.   F.'s  and 
Currents  for  Two-Phase  Alternator 


THREE=PHASE    SYSTEM 

Three  =  Phase     Alternator. 

Consider  three  similar  single- 
phase  armatures  A,  B,  and  C, 
mounted  side  by  side  on  the 

same  shaft  and  revolved  in  the  same  field,  each  armature  having  as 
many  slots  as  there  are  field  poles .  Fix  the  attention  upon  a  certain  ar- 
mature slot  of  A,  and  let  time  be  reckoned  from  the  instant  that  this 
slot  is  squarely  under  an  TV' pole.  Let  t  be  the  time  which  elapses  as  this 


122 


ALTERNATING-CURRENT  MACHINERY 


Fig.   123.    Clock  Diagram  of  Three-Phase 
E.M.F. 'sand  Currents 


armature  slot  passes  from  the  center  of  one  JV  pole  to  the  center  of 
the  next  N  pole.    The  armature  B  is  to  be  so  fixed  to  the  shaft  that 

its  slots  are  squarely  under  the  poles 
at  the  instant  J  t;  and  the  armature 
C  is  to  be  so  fixed  that  its  slots  are 
squarely  under  the  poles  at  the  in- 
stant f  t.  While  a  slot  passes  from 
the  center  of  one  N  pole  to  the 
center  of  the  next  N  pole,  the 
electromotive  force  passes  through 
one  complete  cycle.  Hence,  the 
electromotive  forces  given  by  three 
armatures  arranged  as  above,  will 
be  120  degrees  apart  in  phase,  as  shown  in  Fig.  123,  in  which  the  lines 
A,  B,  and  C  represent  the  respective  electromotive  forces.  The  cur- 
rents given  by  the  armatures  to  three  similar  receiving  circuits  lag 
equally  behind  the  respective  electromotive  forces,  and  are  repre- 
sented by  the  dotted  lines  a,  b,  and  c.  This  combination  of  three 
single-phase  alternators  is  called  a  three-phase  alternator.  In  prac- 
tice the  three  distinct  windings  A,  B,  and  C  are  placed  upon  one 

and  the  same  armature  body. 
For  this  purpose  the  armature 
core  has  three  times  as  many 
slots  as  A,  B,  or  C. 

Fig.  124  shows  the  arrange- 
ment of  the  slots  for  such  a  wind- 
ing. The  slots  belonging  to 
phase  A  are  drawn  in  heavy 
lines,  and  are  marked  ai,  a2, 
etc.  Those  belonging  to  phase 
B  are  shown  dotted,  and  those 
belonging  to  phase  C  are  shown 

Fig.   124.     Arrangement  of  Slots  for  Three-  in    light    lines.       The    A    winding 

Phase  Alternator  Armature  ,  ,  1,1 

would  pass  up  slot  «i,  down  a2, 

up  a3,  etc.;  the  B  winding,  up  61,  down  62,  up  63,  etc.;  and  similarly 
for  winding  C. 

The  windings  A,  B,  and  C  here  described  are  of  the  concen- 
trated type,  having  only  one  slot  per  pole  for  each  winding.     Dis- 


ALTERNATING-CURRENT  MACHINERY 


123 


Fig.   125.     Portion  of  Diagram  of  Windings  for 
Three-Phase  AlternatoV  Armature 


tributed  windings  also  are  frequently  used  for  three-phase  alter- 
nators. Thus  Fig.  125  shows  a  portion  of  a  three-phase  armature 
with  its  A,  B,  and  C  windings  each  distributed  in  two  slots  per 
pole.  The  coils  belonging  to  windings  A,  B,  and  C,  respectively, 
are  differently  shaded  to  distinguish  them.  The  manner  of  connect- 
ing the  coils  of  each  winding  is 
described  on  page  145. 

If  the  three  circuits  of  a 
three-phase  alternator  are  to  be 
entirely  independent,  six  collec- 
tor rings  must  be  used,  two  for 
each  winding;  however,  the  cir- 
cuits may  be  kept  practically 
independent  by  using  four  col- 
lector rings  and  four  mains,  as  shown  in  Fig.  126.  The  main  4  serves 
as  a  common  return  wire  for  the  independent  currents,  in  mains  1, 
2.  and  3.  When  the  three  receiving  circuits  are  equal  in  resist- 
ance and  reactance,  that  is,  when  the  system  is  balanced,  the  three 
currents  are  equal,  and  are  120  degrees  apart  in  phase  (each  cur- 
rent lagging  behind  its  electromotive  force  by  the  same  amount 
as  the  others) ;  and  their  sum  is  at  each  instant  equal  to  zero.  In  this 
case,  main  4>  Fig.  126,  carries  no  current.  Therefore,  main  4  and 
the  corresponding  collector  ring  may  be  dispensed  with,  the  three 
windings  connected  together  at  the  point  A7,  called  the  common 
junction  or  neutral  point.  This  arrangement,  shown  in  the  symmetrical 
diagram,  Fig.  127,  is  called  the  "Y"  or  "star"  scheme  of  connecting 
the  three  windings  or  phases  .A,  B,  and  C. 

Another  scheme  for  connecting  the  three  windings  A,  B,  and 
C  (also  for  balanced  loads), 
called  the  "A"  (delta)  or  "mesh" 
scheme,  is  illustrated  in  Fig.  128. 
Winding  (or  phase)  A  is  connect- 
ed between  rings  3  and  1;  wind- 
ing (or  phase)  B  between  rings 
1  and  2;  and  winding  (or  phase) 
C  between  rings  2  and  3. 

The  direction  in  a  circuit  in  which  the  electromotive  force  or 
current  is  considered  as  a  positive  electromotive  force  or  current, 


Fig.   126. 


Collector  Ring  System  for  Three- 
Phase  Alternator 


124 


ALTERNATING-CURRENT  MACHINERY 


is  called  the  positive  direction  through  the  circuit.  This  direction 
is  chosen  arbitrarily.  The  arrows  in  Figs.  127  and  128  indicate  the 
positive  directions  in  the  mains  and  through  the  windings.  It  must 
be  remembered  that  these  arrows  represent  not  the  actual  direc- 
tions of  the  electromotive  forces  or  currents  at  any  given  instant, 
but  merely  the  directions  of  positive  electromotive  forces  or  currents. 
Thus,  in  Fig.  127,  the  currents  are  considered  positive  when  flowing 
from  the  common  junction  towards  the  collecting  rings,  and  the 
currents  are  never  all  of  the  same  sign. 

Y=Connected  Armatures.  Electromotive  Force  Relations.  We 
shall  consider  the  electromotive  force  between  mains  1  and  2,  Fig. 
127,  to  be  positive,  when  it  tends  to  send  current  through  a  receiving 
circuit  i  from  main  1  to  main  2.  Similarly,  the  electromotive  force 


Fig.  127.     The  "Y"  Scheme  of 

Connecting  Three  Phases  in 

Three-Phase  Alternator 


Fig.   128.     Diagram  of  "A"  Scheme 

forlConnecting  Phases  in  Three- 

Phase  Alternator 


between  mains  2  and  3  is  considered  positive,  from  main  2  to  main 
3;  and  the  electromotive  force  between  mains  1  and  3  is  considered 
positive,  from  main  3  to  main  1.  Passing  through  the  windings 
A  and  B  from  ring  2  to  ring  1,  Fig.  127  (which  is  the  direction  in 
which  an  electromotive  force  must  be  generated  to  give  an  electro- 
motive force  acting  upon  a  receiving  circuit  from  main  1  to  main  2), 
the  winding  A  is  passed  through  in  the  positive  direction,  and  the 
winding  B  in  the  negative  direction.  Therefore,  the  electromotive 
force  from  main  1  to  main  2  is  A—B.  Similarly  the  electromotive 
force  from  main  2  to  main  3  is  B—  C,  and  the  electromotive  force  from 
main  3  to  main  1  is  C—  A.  These  differences  are  shown  in  the  clock 
diagram,  Fig.  129.  The  electromotive  force  between  mains  1  and 
2  (namely,  A—B)  is  30  degrees  behind  A  in  phase,  and  its  effective 


ALTERNATING-CURRENT  MACHINERY 


125 


value  is  2E  cos  30°=  V  3  E,  where  E  is  the  common  value  of  each  of 
the  electromotive  forces  A,  B,  and  C.  Similar  statements  hold 
concerning  the  electromotive  forces  between  mains  2  and  3,  and  those 
between  mains  3  and  1 .  Hence,  the  electromotive  force  beticeen  any 
pair  of  mains  leading  from  a  three-phase  alternator  with  a  Y '-connected 
armature  is  equal  to  the  electromotive  force  generated  per  phase  mul- 
tiplied by  Vs. 

Current  Relations.  In  the  Y  connection,  the  currents  in  the 
mains  are  equal  to  the  currents  in  the  respective  windings  or  arma- 
ture phases,  as  is  evident  from  Fig.  127. 

A=Connected  Armatures.  Electromotive  Force  Relations.  In 
A-connected  armatures  the  electromotive  forces  between  the  mains 


Fig.   129.     Clock  Diagram  of  E.  M.  F.s 
for  Three-Phase  "Y"  Winding 


Fig.   130.     Clock  Diagram  for  Cur- 
rents in  "A"  Connected  Three- 
Phase  Armature 


or  collector  rings  are  equal  to  the  electromotive  forces  of  the  respec- 
tive windings,  as  is  evident  from  Fig.  128. 

Current  Relations.  Referring  to  Fig.  128,  we  see  that  a  positive 
current  in  winding  A  produces  a  positive  current  in  main  1,  and 
that  a  negative  current  in  winding  B  produces  a  positive  current 
in  main  1;  therefore,  the  current  in  main  1  is  a—  b,  where  a  is  the 
current  in  winding  A,  and  b  is  the  current  in  winding  B.  Similarly, 
the  current  in  main  2  is  b—c,  and  the  current  in  main  3  is  c—  a. 
These  differences  are  shown  in  Fig.  130.  The  current  in  main  1 
(namely  a—  b)  is  30  degrees  behind  a  in  phase;  and  its  effective  value 
is  V  37,  where  /  is  the  common  effective  value  of  the  currents, 


126 


ALTERNATING-CURRENT  MACHINERY 


rrta/n  a 


'WOOOOO 


ma/n  3 

Fig.  131.     "Y"  Method  of  Con- 
necting Receiving  Circuits 


a,  b,  c,  in  the  different  phases.     Similar  statements  hold  for  the 
currents  in  mains  2  and  3;  so  that  the  current  in  each  main  from 
a  A-connected  armature  is  V  3  times  the  current  in  each  winding. 
Receiving  Circuits  to  Three=Phase  Mains.    Dissimilar  Circuits 
(Unbalanced  System) .    WThen  the  receiv- 
ing   circuits   which    take   current   from 
three-phase  mains  are  dissimilar,  that  is, 
do  not  each  have  equal  resistance  and 
reactance,  four  mains  should  be  employed, 
as  indicated  in   Fig.    126;   each  receiv- 
ing circuit  being   connected  from  main 
4  to  one  of  the  other  mains.    A  com- 
mon example  of  an  unbalanced  system 

is  where  a  mixed  load  of  induction  motors  and  incandescent  lamps  is 
connected  unsymmetrically  to  three-phase  receiving  mains.  It  is, 
however,  desirable  to  keep  the  three  windings  A,  B,  and  C  of  the 
alternator  almost  equally  loaded;  and  in  practice  the  receiving  cir- 
cuits are  so  disposed  as  to  satisfy  this  condition  as  nearly  as  possible. 
Similar  Circuits  (Balanced  System).  When  three-phase  cur- 
rents are  used  to  drive  induction  motors,  synchronous  motors,  or 
rotary  converters,  each  one  of  these  machines  takes  current  equally 
from  the  three  mains;  and  since  three-phase  currents  are  utilized 
chiefly  in  the  operation  of  the  machines  mentioned,  the  system  is 
usually  balanced.  In  this  case  three  mains  only  are  employed,  and 
each  receiving  unit  has  three  similar  receiving  circuits  connected 
to  the  mains  according  to  either  the  Y  or  the  A  method.  The  Y  method 
of  connecting  receiving  circuits  is  shown  in  Fig.  131.  One  terminal 

of  each  receiving  circuit  is  connected 
to  a  main,  and  the  other  terminals  are 
connected  together  at  the  neutral  point 
N.  In  this  case  the  current  in  each  re- 
ceiving circuit  is  equal  to  the  current  in 
the  main  to  which  it  is  connected.  The 
electromotive  force  between  the  termi- 
nals of  each  receiving  circuit,  as  A,  is 
equal  to  —  =  ,  where  E  is  the  electromotive  force  between  any  pair 


ma/n    \ 


nia/n 


Fig.   132.     "A"  Method  of  Connect- 
ing Receiving  Circuits 


of  mains. 


ALTERNATING-CURRENT  MACHINERY 

TABLE  HI 
A-  and  Y -Connection  Data  in  Mains 


127 


A  connection  
Y  connection  

E.  M.  F.  between 
Mains 

Current  in  Each 
Main 

Power  Rating 

'*' 

VT/. 

The  A  method  of  connecting  receiving  circuits  is  shown  in 
Fig.  132.  Here  the  three  receiving  circuits  are  connected  between 
the  respective  pairs  of  mains;  the  electromotive  force  acting  on 
each  receiving  circuit  is  the  electromotive  force  between  the  mains; 

and   the   current  in  each  receiving  circuit  is ,-  where  I  is  the 

current  in  each  main.  *     3 

Examples.  1.  The  three  windings  or  phases  of  a  three-phase  induc- 
tion motor  are  Y-connected  to  three-phase  mains.  The  voltage  between 
mains  is  500,  and  each  main  delivers  25  amperes  to  the  motor.  It  is  required 
to  find  the  current  in  each  phase  of  the  motor,  and  the  electromotive  force 
acting  on  each  phase  of  the  motor. 

Solution.  Since  the  windings  are  Y-connected,  the  current  in  each  is 
the  same  as  the  current  in  each  main,  namely,  25  amperes;  and  the  electro- 

500 
motive  force  acting  on  each  phase  of  the  motor  winding  is  -^=. ,  or  288.7  volts. 

The  power  input  is  P  =  V  3   X  500  X  25  =  21650  watts.        3 

2.  The  three  phases  of  the  above  three-phase  induction  motor  are  A-con- 
nected  to  three-phase  mains.  The  voltage  between  mains  in  288.7,  and  the 
current  in  each  main  is  43.3  amperes.  It  is  required  to  find  the  current  in 
each  phase  of  the  motor,  and  the  electromotive  force  acting  on  each  phase  of 
the  motor. 

Solution.  Since  the  windings  are  A-connected,  the  electromotive  force 
acting  on  each  phase  is  the  same  as  the  voltage  between  the  mains,  namely  ^ 

43  3 
288.7  volts;  and  current  in  each  phase  of  the  motor  is  —  —   or  25  amperes 

_  V  3 

The  power  input  is  P=  V  3   X  288.7  X  43.3  =  21650  watts,  the  same  as  before. 

Summary  of  Electromotive  Force  and  Current  Relations  for 
A  and  Y  Connections.  Let  Ew  be  the  rated  electromotive  force  of 
each  winding,  and  /„,  the  rated  full-load  current  output  of  each  phase 
of  the  winding  of  a  three-phase  alternator,  then,  for  a  generator  with 
non-inductive  load,  the  data  is  as  given  in  Table  III. 

Let  E  be  the  electromotive  force  between  mains  of  a  three- 


128 


ALTERNATING-CURRENT  MACHINERY 


TABLE  IV 
and  Y=Connection  Data  in  Receiving  Circuits 


E.  M.  F.  between  Ter- 
minals of  Each  Re- 
ceiving Circuit 

Current  in  Each 
Receiving 
Circuit 

Total  Power 
Input 

A  connection 

E 

7 

V1T7?7 

E 

VT 

Y  connection.  . 

_l~ 

7 

V  3    El 

"V3 

phase  system,  and  let  /  be  the  current  in  each  main,  then,  for  three 
receiving  circuits,  the  data  is  as  given  in  Table  IV. 

The  permissible  power  output  or  rating  of  a  three-phase  alter- 
nator is  the  same  whether  its  armature  windings  are  Y-connected 
or  A-connected. 

The  power  output  of  a  three-phase  generator  is  V  3  X  electro- 
motive force  between  mainsX  current  in  one  mainX  power  factor  of  the 
receiving  circuits. 

MEASUREMENT  OF  POWER 

In  alternating-current  circuits,  power  cannot  in  general  be 
measured  by  means  of  an  ammeter  and  a  voltmeter,  as  in  the  case 
of  direct  current,  because  the  power  expended  is  generally  less  than 
the  product  of  effective  electromotive  force  and  effective  current  on 
account  of  the  difference  in  phase  between  the  electromotive  force 
and  the  current. 

A  well-designed  wattmeter  is  the  standard  instrument  for 
measuring  power  in  alternating-current  circuits,  and  the  methods 
involving  its  use  are  the  most  generally  satisfactory. 

NOTE. — The  discussion  of  the  wattmeter  given  on  page  78  applies  pri- 
marily to  the  use  of  the  wattmeter  for  the  measurement  of  power  delivered 
in  single-phase  systems. 

The  several  circuits  of  a  polyphase  system  are  often  entirely  sep- 
arate and  independent;  and  in  such  cases  the  total  power  delivered  to  a 
receiving  apparatus  is  found  by  measuring  the  power  delivered  to  each 
separate  receiving  circuit.  The  total  power  delivered  is  the  sum  of  the 
amounts  delivered  to  the  different  receiving  circuits. 

In  order  to  measure  the  power  delivered  to  one  of  the  receiv- 
ing circuits  of  a  polyphase  system,  the  current  coil  of  the  watt- 
meter is  to  be  connected  in  series  with  this  receiving  circuit,  and 


ALTERNATING-CURRENT  MACHINERY  129 

the  pressure  (or  voltage)  coil  of  the  wattmeter  is  to  be  connected 
between  the  terminals  of  this  receiving  circuit.  In  some  cases  this 
connection  of  the  voltage  coil  cannot  be  made,  because  on  ter- 
minal of  the  receiving  circuit  may  be  out  of  reach  in  th,e  interior 
of  the  apparatus. 

Balanced  Systems.  In  general,  the  several  circuits  which 
receive  current  and  power  from  polyphase  mains  are  more  or  less 
unlike  in  both  resistance  and  reactance,  and  take  different  amounts 
of  current  and  power  from  the  mains.  It  is,  however,  desirable 
that  the  several  receiving  circuits  be  alike,  so  that  they  may  take 
equal  currents  and  equal  amounts  of  power  from  the  mains.  When 
this  condition  is  realized,  the  system  is  said  to  be  balanced. 

For  example,  when  independent  groups  of  lamps  are  supplied 
from  polyphase  mains,  each  group  taking  current  directly,  or  through 
a  transformer,  from  one  phase  of  the  polyphase  system,  the  system 
is  said  to  be  unbalanced  when  the  number  of  lamps  is  not  the  same 
in  the  several  groups.  In  general,  the  supply  of  power  to  several 
separate,  independent,  and  unrelated  receiving  circuits,  such  as 
independent  groups  of  lamps  and  single-phase  motors,  leads  to  the 
unbalancing  of  a  polyphase  system.  Apparatus,  such  as  polyphase 
induction  motors,  synchronous  motors,  and  rotary  converters  which 
are  especially  designed  to  take  power  from  polyphase  mains,  is  always 
provided  with  two  or  three  similar  receiving  circuits  so  as  to  take 
equal  amounts  of  current  and  power  from  each  phase  of  the  system. 
When  such  polyphase  apparatus  takes  power  from  polyphase  mains 
only,  the  system  is  always  very  nearly  balanced. 

If  a  polyphase  system  were  exactly  balanced  it  would  be  suffi- 
cient to  measure  the  power  delivered  by  one  phase  only;  but  since 
a  balanced  condition  of  a  system  is  seldom  exactly  realized  in  prac- 
tice, there  may  be  considerable  error  introduced  by  assuming  that 
a  system  is  balanced,  and  by  calculating  the  total  power  from  the 
wattmeter  reading  of  power  delivered  by  one  phase  only. 

In  balanced  or  approximately  balanced  polyphase  systems, 
the  measurement  of  power  by  use  of  a  single  wattmeter  is  best 
accomplished  by  special  arrangement  of  connections  as  follows: 

Three-Wire  Two-Phase  Systems.  The  current  coil  of  the  watt- 
meter should  be  connected  in  the  middle  main  as  shown  in  Fig.  133. 
After  a  reading  is  taken  with  the  pressure  coil  connected  between 


130 


ALTERNATING-CURRENT  MACHINERY 


middle  and  lower  mains,  this  connection  is  quickly  changed  to  the 
upper  main,  as  indicated  by  the  dotted  line,  and  the  wattmeter 
again  read.  The  sum  of  the  two  successive  readings  gives  the  total 
power.  If  this  method  is  used,  the  system  should  not  only  be  bal- 
anced, but  no  change  in  the  load  should  occur  between  readings. 


7C7S6   A 

rece/\ 

7S<?    B 

'  000000  ^i  '  — 
\aa2QCQMJb-' 

rece/Vf 

Fig.   133.     Diagram  of  Power  Connection  for  Three- Wire 
Two-Phase  Induction  Motor 

For  example,  the  power  taken  by  a  two-phase  induction  motor 
is  measured  by  a  wattmeter  connected  as  shown  in  Fig.  133.  When 
the  wattmeter  is  connected  as  shown  by  the  full  line  in  the  figure, 
it  reads  9,900  watts.  When  the  wattmeter  is  connected  as  shown 
by  the  dotted  line,  it  reads  1,415  watts.  The  total  watts  delivered 
are,  therefore,  11,315  watts,  the  two  phases  of  the  motor  being 
assumed  to  be  balanced.  Each  phase  of  the  motor  receives,  there- 
fore, -  -  =  5,657  watts.  The  current  delivered  to  each  phase, 

as  measured  by  an  alternating-current  ammeter  is  32.14  amperes; 
and  the  electromotive  force  acting  on  each  phase  of  the  motor, 
that  is,  between  the  terminals  of  each  receiving  circuit,  is  220  volts. 
The  apparent  power  (volt-amperes)  delivered  to  each  phase  is  220 

main   \  . 


main   3 


W 


Fig.  134.     Diagram  of  Power  Connection 
for  Three-Wire  Three-Phase  System 


voltsX32.14  amperes=  7,071  apparent  watts;  and  the  power  factor 
of  each  receiving  circuit  is 


receiving  circuit  is 

5,657  watts 


7.071  apparent  watts 


,  or  0.80 


]  ALTERNATING-CURRENT  MACHINERY  131 

The  two  readings  of  a  wattmeter,  connected  as  shown  in  Fig. 
133,  are  unequal,  even  though  the  receiving  circuits  are  balanced, 
because  of  the  effect  of  lagging  currents;  or,  in  other  words,  because 
the  two  receiving  circuits  are  inductive. 

It  is  to  be  carefully  noted  that,  in  general,  neither  reading  of 
the  wattmeter  measures  the  power  delivered  to  either  one  of  the 
receiving  circuits. 

Three-Wire  Three-Phase  Systems.  The  current  coil  of  the 
wattmeter  should  be  connected  in  series  with  one  (any  one)  of  the 
three  mains,  as  shown  in  Fig.  134.  After  one  reading  of  the  watt- 
meter is  taken  with  the  voltage  coil  connected  to  main  2t  as  indicated 
by  the  full  line,  the  connection  is  quickly  changed  to  main  1,  as 
indicated  by  the  dotted  line,  and  a  second  reading  of  the  watt- 


currenf  co/7 


to  supp/y  ma/ns 


Phase    B 


current  co/7 

Fig.   135.     Power  Connection  of  Four- Wire  Two-Phase  System 

meter  is  taken.  The  total  power  delivered  to  the  three  similar 
(that  is,  balanced)  phases,  is  equal  to  the  sum  of  the  two  readings. 

Unbalanced  Systems.  In  general,  any  receiving  apparatus  is 
sufficiently  unbalanced  to  require  the  measurement  of  power  to  be 
made  on  the  assumption  that  the  receiving  circuits  are  unbalanced. 

Four-Wire  Two-Phase.  When  four  mains  are  used,  two  for 
each  separate  phase,  then  two  wattmeters  are  required,  one  for 
measuring  the  power  delivered  by  each  phase.  Each  of  these  watt- 
meters is  connected  exactly  as  in  the  case  of  single-phase  delivery 
of  power,  as  shown  in  Fig.  135.  The  sum  of  the  readings  Wi^-W* 
of  the  two  wattmeters  gives  the  total  power  delivered.  Two  readings 
should,  of  course,  be  taken  as  nearly  simultaneously  as  possible. 


132 


ALTERNATING-CURRENT  MACHINERY 


Three-Wire  Two-Phase.    When  a  two-phase  system  is  balanced 
or  unbalanced  and  has  three  supply  mains,  one  main  acting  as  the 


ma/n  \ 


current  coi/ 


to  /oaet 
common  return 


to  /oaa? 

vo/tage  coil 

oooooxxr — I 

,  000000  Jwa 
current  co// 

Fig.   136.     Diagram  of  Power  Connection  for 
Three- Wire  Two-Phase  System 

common  return  for  the  other  two,  then  the  arrangement  shown  in 
Fig.  136  gives  the  best  results.  The  total  power  delivered  to  the 
receiving  circuit  is  the  sum  W\-\-  W%  of  the  readings  of  the  two  watt- 
meters. The  readings  should  be  taken  as  nearly  simultaneously 
as  possible. 


Phase  A 


to  /oad 


Phase  B 


-^to  Jo  act 


W3 


Phase   C 


to  /octet 


Fig.  137.     Diagram  of  "Power  Connection  for  Six-Wire 
Three-Phase  System 

Six-Wire  Three-Phase.    When  six  mains  are  used,  two  for  each 
separate  phase,  then  three  wattmeters  are  required,  one  for  measur- 


ALTERNATING-CURRENT  MACHINERY  133 

ing  the  power  delivered  by  each  phase.  Each  of  these  wattmeters 
is  connected  exactly  as  in  the  case  of  single-phase  delivery  of  power, 
as  shown  in  Fig.  137. 

In  practice,  six  wires  are  never  used  for  three-phase  systems 
on  account  of  complications  and  the  excessive  amount  of  copper 
required. 

Three-Wire  Three-Phase.  When  three  mains  are  used  in  a 
three-phase  system,  two  wattmeters  are  sufficient  for  the  com- 
plete measurement  of  the  power  delivered  to  any  three-phase  receiv- 
ing unit,  whether  the  receiving  circuit  is  balanced  or  unbalanced,  or 
whether  it  is  connected  Y  or  A. 

Fig.  138  shows  two  wattmeters  connected  for  measuring  the 
power  delivered  to  a  A-connected  three-phase  receiving  system. 
The  algebraic  sum  of  the  readings  of  the  two  wattmeters  gives  the 


main    \ 


ma/n 


W 


Fig.  138.     Diagram  of  Power  Connection  for  Three- Wire 
Three-Phase  System 

total  power  delivered  independent  of  balance  or  lag.  When  the 
current  lag  in  the  circuit  is  less  than  60  degrees,  i.  e.,  when  the  power 
factor  is  greater  than  0.5,  then  the  arithmetical  sum  of  the  readings 
of  the  two  wattmeters  gives  the  total  power.  But  if  the  lag  is  greater 
than  60  degrees,  i.  e.,  when  the  power  factor  is  less  than  0.5,  the 
relation  of  the  currents  in  the  current  coil  and  pressure  coil  of  one  of 
the  wattmeters  causes  it  to  give  a  negative  reading;  hence  the  arith- 
metical difference  of  the  readings  of  the  two  instruments  gives  the 
power,  i 

There  may  be  a  difficulty  in  determining  which  condition  exists 
in  some  cases,  especially  when  the  power  delivered  to  partially  loaded 
induction  motors  whose  power  factor  is  low,  is  to  be  measured. 
In  such  cases  one  may  determine  whether  the  sum  or  difference 
of  readings  is  to  be  taken  by  interchanging  the  position  of  the  in- 


134  ALTERNATING-CURRENT  MACHINERY 

struments  without  changing  the  relative  connections  of  their  cur- 
rent and  pressure  coils.  If  the  deflections  of  both  pointers  are  now 
reversed,  the  difference  of  the  original  readings  gives  the  true  power, 
but  if  the  deflections  are  in  the  same  direction  as  before,  the  sum  of 
the  original  deflections  is  the  correct  power. 

To  prove  the  accuracy  of  these  deductions,  let  the  positive  direc- 
tion in  the  mains  1  and  2  and  in  the  three  receiving  circuits  be  chosen 
as  indicated  by  the  arrows  in  Fig.  138.  These  directions  are  chosen 
symmetrically  with  respect  to  the  two  wattmeters.  Let  the  instan- 
taneous currents  in  the  receiving  circuits  be  i',  i"  ,  and  i'",  as  shown 
in  the  figure.  Let  a  be  the  instantaneous  current  in  main  1,  and  let 
b  be  the  instantaneous  current  in  main  2.  Then,  from  the  arbitrary 
choice  of  signs, 


The  reading  Wf  of  the  upper  wattmeter  is  equal  to  the  average 
value  of  the  product  of  the  current  a,  which  flows  through  the  cur- 
rent coil  of  the  instrument,  and  the  electromotive  force  e',  which 
is  acting  upon  the  pressure  coil  of  the  wattmeter.  That  is 

W=  average  (aef) 

Similarly,  the  reading  W"  of  the  lower  wattmeter  gives 

W"=  average  (be"). 

Substituting  the  above  values  of  a  and  b  in  the  expressions  for  W 
and  W",  and  adding  results,  we  have 

W'+W"=  average  (e'i')+  average  (e"  i")+  average  (e'-e")  i'"> 
But,  from  the  figure,  e'—  e"=e"f;  hence 

W'+W"=  average  (e'  i')+average  (e"  i")+  average  (e'n  i'"} 

Although  a  formal  proof  of  the  principle  of  the  two-wattmeter 
method  has  not  been  given  for  a  Y-connected  circuit,  it  is  not  neces- 
sary to  show  independently  that  it  holds  for  both  cases.  A  little 
consideration  will  show  that  if  the  electromotive  forces  acting  be- 
tween the  three  wires,  the  currents  flowing  in  them,  and  their  phase 
relations  are  given,  there  is  then  a  perfectly  definite  amount  of 
power  transmitted  along  the  three  lines,  and  it  is  quite  immaterial 
whether  this  power  is  being  delivered  to  circuits  connected  A  or  Y. 


ALTERNATING-CURRENT  MACHINERY  135 

ARMATURE  WINDINGS 

In  general,  any  direct-current  armature  winding  may  be  used 
for  the  armature  of  an  alternator;  but  the  desirability  of  generating 
comparatively  high  voltages  in  the  armature  so  as  to  avoid  the 
use  of  step-up  transformers,  makes  it  necessary  to  abandon  the 
styles  of  winding  best  suited  to  direct-current  machines,  and  to 
use  windings  specially  adapted  to  the  conditions  of  alternating- 
current  practice. 

Comparing  the  armature  windings  used  for  direct-current 
machines  with  those  for  alternators,  we  find,  first,  that  all  the  re- 
entrant (or  closed-coil)  direct-current  windings  must  necessarily 
be  either  two-circuit  or  multiple-circuit  windings,  i.  e.,  they  must 
have  at  least  two  paths  in  parallel  through  the  armature  between 
brushes;  and  second,  that  the  armatures  of  alternators  (and  syn- 
chronous motors)  may,  and  generally  do,  from  practical  considera- 
tions, have  one-circuit  windings,  i.  e.,  windings  having  one  cir- 
cuit per  phase.  It  follows,  therefore,  that  any  direct-current  wind- 
ing may  be  used  for  alternating-current  machines;  but  the  con- 
verse statement,  that  any  alternating-current  winding  may  be  used 
for  direct-current  machines,  is  not  true  in  general.  In  other  words, 
the  windings  of  alternating-current  armatures  are  essentially  non- 
re-entrant  (or  open-circuit)  windings.  The  only  exceptions  are  the 
A-connected  (or  mesh-connected)  polyphase  windings,  and  the 
short-circuited  windings  of  "squirrel-cage"  induction  motors,  both 
of  which  are  re-entrant  (or  closed-circuit)  windings.  The  A-con- 
nected polyphase  windings  are,  therefore,  the  only  windings  that 
can  be  used  for  the  armatures  of  rotary  converters. 

In  the  type  of  winding  generally  employed  for  alternators,  a 
number  of  distinct  coils  are  arranged  on  the  armature;  in  these  coils 
alternating  electromotive  forces  are  induced  as  they  pass  the  field- 
magnet  poles,  and  the  several  coils  are  connected  in  series  between 
the  collecting  rings. 

Classification.  According  to  Shape  oj  Core.  Armatures  for 
alternators,  just  as  in  the  case  of  direct-current  machines,  may  be 
divided  into  drum  armatures;  ring  armatures;  and  disk  armatures. 
Of  these  the  ring  and  the  disk  armatures  are  seldom  used  in 
America  although  they  are  to  some  extent  adopted  in  European 
practice. 


136  ALTERNATING-CURRENT  MACHINERY 

The  ring  and  the  disk  types  of  the  armature  are  mechanically 
less  stable  than  the  drum  type;  and  the  ring  armature,  moreover, 
other  things  being  equal,  requires  more  wire  to  be  wound  upon  it  for 
a  given  output  than  in  the  case  of  the  drum  armature,  and  pos- 
sesses, therefore,  a  greater  inductance  than  the  latter  type.  Drum 
armatures,  whether  the  alternators  are  of  the  revolving  or  station- 
ary armature  type,  have  laminated  iron  cores  similar  in  construc- 
tion to  the  armature  cores  for  direct-current  machines.  Disk 
armatures,  on  the  other  hand,  are  usually  made  up  without  iron, 
thus  introducing  constructional  difficulties. 

According  to  Construction  of  Core.  With  reference  to  the  con- 
struction of  their  cores,  the  armatures  of  alternators  may  be  classi- 
fied, as  in  the  case  of  direct-current  machines,  into  smooth-core 
armatures  and  toothed-core  armatures. 

In  the  smooth-core  armature  the  conductors,  arranged  in  flat 
coils,  lie  on  the  surface  of  the  armature  core,  and  the  coils  in  some 
cases  are  bent  down  over  the  ends  of  the  core,  where  they  are  fastened 
by  end  plates  or  by  blocks  of  wood  or  fiber.  The  spaces  in  the 
centers  of  the  coils  are  filled  with  wooden  blocks  either  screwed  to 
the  cores  or  held  in  place  by  the  binding  wires.  In  other  cases 
the  coils  are  flat  or  "pancake"  shaped,  and  of  the  same  length  as 
the  armature  core.  In  the  latter  case  they  are  laid  upon  the  cylin- 
drical surface  of  the  armature  core,  and  are  securely  bound  with 
wire  bands. 

The  form  of  the  wave  of  electromotive  force  produced  by 
smooth-core  armatures  is  very  nearly  harmonic  (sinusoidal)  or 
slightly  flat-topped.  The  inductance  of  a  smooth-core  (or  surface- 
wound)  armature  is  considerably  less  than  that  of  a  toothed-core 
armature.  Although  much  used  in  earlier  designs,  the  smooth- 
core  armatures  owing  largely  to  their  comparatively  weak  mechan- 
ical structure,  have  been  superseded  in  modern  practice  by  the 
toothed-core  constructions, 

One  or  another  of  the  forms  of  toothed-core  armature  is  now 
almost  universally  used  in  practice.  The  conductors  are  laid  in 
slots  or  grooves,  the  sides  and  bottom  of  which  are  first  carefully 
insulated  by  troughs  of  mica-canvas,  micanite,  or  other  suitable 
insulating  material.  The  insulated  conductors  (cotton  covered) 
are  generally  wound  into  coils  on  "formers,"  each  coil  being  care- 


ALTERNATING-CURRENT  MACHINERY  137 

fully  taped,  and  then  impregnated  with  insulating  compound  (or 
varnish).  The  coils  are  then  thoroughly  dried  by  baking  in  ovens. 
The  conductors  being  enclosed  in  slots  between  teeth  which 
project  more  or  less  over  the  conductors,  the  toothed-core  type  is 
often  called  iron-clad.  This  construction  has  three  great  advantages 
over  the  smooth-core  type: 

(a)  It  allows  the  length  of  air  gap  from  iron  of  pole-face  to  iron  of  arma- 
ture core  to  be  reduced  to  a  minimum;  just  enough  for  mechanical 
clearance.  Other  things  being  equal,  this  means  a  saving  in  the  cop- 
per required  to  magnetize  the  field. 

(b)It  protects  the  embedded  conductors  from  injury. 

(c)  It  affords  an  admirable  way  of  supporting  and  securing  the  conductors 
firmly  in  place  against  the  action  of  centrifugal  force;  and,  further,  it 
shields  the  conductors  almost  completely  from  the  racking  action  of 
the  magnetic  drag  due  to  the  magnetic  field. 

The  shape  and  number  of  the  slots  in  a  toothed  armature  core 
have  a  marked  effect  on  the  shape  of  the  electromotive  force  wave, 
and  upon  the  regulation  of  the  alternator.  The  shape  of  the  wave  is 
affected  by  the  distribution  of  the  magnetic  flux  in  the  air  gap. 
The  regulation  is  affected  by  the  inductance  of  the  armature  coils, 
and  the  inductance  depends  on  the  number  and  shape  of  the  arma- 
ture slots. 

Fig.  139  shows  a  portion  of  an  armature-  r"CA\  \J  u  //"? 
core  disk  or  stamping  for  a  12-pole,  uni-tooth  \  ' 

(one  slot  per  pole  per  phase),  three-phase 
alternator.  The  armature  winding  adapted 
to  this  uni-tooth  core  is,  of  course,  the  uni-  Fig.  139  Portion  of  stamp- 

t  ing  for  Twelve-Pole  Um- 

coil  or  concentrated  winding.     The  armature          TootAiSnato?hase 
coils  are  held  in  place  in  the  slots  by  wedges 

of  wood  or  fiber  driven  in  from  the  ends  of  the  core  and  fitting 
into  notches  near  the  tops  of  the  teeth,  as  shown  in  the  figure. 
This  construction,  now  almost  universally  adopted  by  manufact- 
urers, avoids  the  necessity  for  any  binding  wire  on  the  arm- 
ature core. 

Alternators  with  uni-tooth  armature  cores  are  characterized 
by  large  armature  inductance  and  by  peaked-wave  shapes  of  the 
induced  electromotive  forces;  also  by  marked  variations  in  the 
shape  of  the  wave  of  induced  electromotive  force,  according  to  the 
magnitude  and  power  factor  of  the  load. 


138  ALTERNATING-CURRENT  MACHINERY 

On  account  of  their  comparatively  large  inductance,  uni-tooth 
armature  constructions  require  a  relatively  large  increase  in  the 
field-exciting  current  in  passing  from  no  load  to  full  load  output. 
In  other  words,  regulation  is  poorer  than  for  multi-tooth  armature 
cores. 

According  to  present  practice  in  design,  the  great  majority  of 
alternators  are  constructed  with  armature-core  stampings  having 
two  or  three  or  more  slots  per  pole  per  phase.  Fig.  140  shows  a 
portion  of  an  armature-core  stamping  for  a  12-pole,  three-phase 
alternator.  It  has  three  slots  per  pole  per  phase.  The  slots  are 
open,  which,  together  with  the  distributed  (multi-coil)  type  of 
winding,  results  in  a  low  armature  inductance.  This  means  that  a 
relatively  small  increase  in  the  field-exciting  current  is  required  in 
passing  from  no  load  to  full  load  output. 

Alternators   with    multi-tooth    armature    cores    are   especially 
adapted  for  long-distance  transmission  where  step-up  transformers 
are  used.     The  regulation  is  better  than  with  the  uni-tooth  core 
construction;  and  the  wave  shape  of  the  electromotive  force  gener- 
ated by  the  distributed  winding  approaches  a  sine  wave,  which  is 
the  best  wave  shape  for  the  long-distance  transmission  of  power. 
This  is  because  of  the  fact  that  the  nearer  a 
f\t\T\I\fl/l/]/)/7/i     giyen  curve  of  electromotive  force  approaches 
V^  /     a  sme  curve,  the  less  the  likelihood  of   a  dan- 

\  /        gerous  rise  of  voltage  (resonance)  occuring   at 

V -•>/          the  distant  end  of  a  long  transmission  line  be- 

cause  of  the  capacity  (condenser)  effect.  A 
long  transmission  line  has  a  series  of  frequen- 
cies of  electrical  oscillation  just  as  a  stretched 
violin  string  has  a  series  of  frequencies  of  mechanical  vibration. 
If  the  frequency  of  the  current  delivered  by  an  alternator  hap- 
pens to  coincide  with  any  of  these  "proper"  frequencies  of  the 
transmission  line,  the  line  will  have  violent  electrical  oscillations 
set  up  in  it,  and  excessive  voltages  will  occur  at  certain  points 
along  the  line.  A  sine  wave  of  electromotive  force  has  only  one 
frequency.  Any  other  kind  of  an  alternating  electromotive  force 
is  composed  of  a  series  of  sine  waves  of  ascending  frequencies  (the 
harmonics  or  over-tones  in  music),  all  combined  into  a  resultant 
wave  form.  There  is,  therefore,  more  danger  that  one  out  of  all 


ALTERNATING-CURRENT  MACHINERY  139 

the  frequencies  may  coincide  with  one  of  the  "proper"  line  fre- 
quencies than  that  the  single  frequency  of  a  sine  wave  may  so 
coincide. 

According  to  Progression  of  Winding.  With  reference  to  the 
progress  of  the  winding  from  slot  to  slot,  armature  windings  may 
be  divided  into  spiral  or  ring  windings;  lap  windings;  and  wave 
windings.  These  terms  have  the  same  meaning  when  applied  to 
alternator  windings  as  they  do  when  applied  to  the  windings  of 
direct-current  machines. 

According  to  Disposition  of  Coils.  With  reference  to  the  dis- 
position of  the  coils  around  the  periphery  of  the  core,  we  have  to 
distinguish  between  two  general  classes,  viz,  concentrated,  or  uni-coilt 
windings,  and  distributed,  or  multi-coil,  windings. 

Concentrated,  or  uni-coil,  windings,  as  the  name  implies,  con- 
sist of  one  coil  per  pole  per  phase.  The  armature  conductors  are 
thus  grouped  in  bundles,  and  usually  placed  in  slots,  there  being 
one  slot  per  pole  for  each  phase. 

Examples  of  concentrated  windings  are  illustrated  in  Figs.  1  and 
102,  in  which  the  armature  conductors  are  shown  as  lying  in  one 
slot  per  pole.  Fig.  1  shows  adjacent  sides  of  two  different  armature 
coils  lying  in  one  slot.  In  some  cases,  each  slot  is  filled  by  one 
side  of  a  single  armature  coil,  giving  one  slot  per  phase  per  pair 
of  poles.  Such  windings  are  sometimes  called  "half-coiled"  or 
"hemi-tropic." 

Distributed,  or  multi-coil,  windings  consist  of  several  coils  per 
pole  per  phase.  The  armature  conductors  are  distributed  in  two 
or  more  slots  per  pole  per  phase. 

Examples  of  distributed  windings  are  shown  in  Figs.  120  and 
125.  Fig.  120  shows  an  end  view  of  a  two-phase  winding  distributed 
in  two  slots  per  pole  per  phase.  Fig..  125  shows  an  end  view  of  a 
three-phase  winding  distributed  in  two  slots  per  pole  per  phase. 

Concentrated  windings  .are  less  expensive  to  make;  and  they 
give  a  greater  effective  electromotive  force  (at  zero  load)  for  a  given 
number  of  conductors,  other  things  being  equal,  than  distributed 
windings.  This  is  on  account  of  the  fact  that  all  the  conductors  of 
a  concentrated  winding  cut  the  field  flux  simultaneously,  while  the 
various  conductors  of  a  distributed  winding  do  not  cut  the  field 
flux  simultaneously. 


140  ALTERNATING-CURRENT  MACHINERY 

Concentrated  windings  have  greater  inductance  than  distrib- 
uted windings  for  the  same  total  number  of  conductors,  and  also 
cause  a  greater  armature  reaction  for  a  given  current  than  distrib- 
uted windings  do.  Consequently  the  terminal  electromotive  force 
of  an  alternator  falls  off  more  with  a  concentrated  winding  than 
with  a  distributed  winding,  when  the  current  output  is  increased. 


Fig.  141.     Completed  Armature  with  Strap  Winding,  Four  Slots  Per  Phase 

Therefore,  an  alternator  with  a  concentrated  winding  has  a  poorer 
(higher)  regulation  than  an  alternator  with  a  distributed  winding; 
and  although  a  concentrated  winding  may  give  a  higher  electro- 
motive force  at  zero  load,  it  may  actually  give  a  lower  electromo- 
tive force  at  full  load. 


ALTERNATING-CURRENT  MACHINERY 


141 


According  to  Form  of  Conductor.  According  to  the  form  of  the 
conductors  used,  armature  windings  may  be  divided  into  three 
classes,  viz,  wire  winding;  strap  winding;  and  bar  winding. 


Fig.  142.     Portion  of  Bar-Wound  Armature,  One  Bar  Per  Slot 

Wire  ivinding,  which  is  usually  employed  in  high- voltage 
machines  of  low-current  output,  consists  of  machine-wound  coils, 
which  are  entirely  formed  and  insulated  before  being  placed  in  the 
armature  slots. 


142  ALTERNATING-CURRENT  MACHINERY 

Strap  winding  is  used  for  machines  of  lower  voltage  and  of 
greater  current  output,  and  it  consists  of  copper  strap,  forged  into 
the  required  shape  and  carefully  insulated. 

Both  the  wire  and  strap  windings  are  placed  in  the  slots  with- 
out any  mechanical  bending,  thus  preventing  damage  to  the  insu- 
lation. In  armature  core's  having  the  slots  partially  closed,  the 
winding  is  slipped  in  from  the  end;  but  in  cores  having  open  slots, 
wedges  of  hard  fiber  secure  the  coils  in  place. 

Fig.  141  is  an  illustration  of  strap  winding  distributed  in  four 
slots  per  pole  per  phase.  The  completed  armature,  ready  for  direct 
connection  to  a  steam  engine,  is  shown  in  the  figure,  and  is  intended 
to  revolve  inside  of  a  stationary  field-magnet  structure.  The 
four  collector  rings  indicate  that  the  armature  is  wound  for  two 
phases.  It  is  manufactured  by  the  Westinghouse  Electric  Company. 
Bar  windings  are  held  in  place  by  the  overhanging  tips  of  the 
teeth.  The  bars,  after  being  carefully  insulated,  are  slipped  into 

the  slots  from  one  end  of  the 
armature.  The  end  connections 
of  the  bar  winding  are  bolted  and 
soldered  to  the  bars  after  the  con- 
ductors are  in  place. 

Bar  windings  are  usually  ar- 
ranged with  either  one  or  two  bars 
per  slot.  There  are  no  band  wires 
on  the  armature  core. 

Fig.  142  shows  a  portion  of  a  bar- 
Fig.  143.    Concentrated  single-Phase        wound  revolving  armature  having 

Armature  Winding 

one  bar  per  slot. 

Single=Phase  Windings.  Fig.  1  shows  a  common  type  of 
single-phase  winding  having  one  coil  per  pole.  Fig.  143  shows 
another  type  of  concentrated  single-phase  winding,  having  one  coil 
to  each  pair  of  poles  or  one  slot  per  pole.  The  sketch  b  is  a  sectional 
view  of  a  portion  of  the  armature  core,  showing  one  of  the  slots 
containing  the  conductors  forming  one  side  of  a  single  armature 
coil  and  standing  opposite  to  a  field  pole.  In  the  main  diagram 
the  heavy  sector-shaped  figures  represent  the  coils,  and  the  light 
lines  represent  the  connections  between  the  terminals  of  the  coils. 
The  radial  parts  of  the  sector-shaped  figures  represent  the  portions 


ALTERNATING-CURRENT  MACHINERY 


143 


of  the  coils  that  lie  in  the  slots,  and  the  curved  parts  represent  the 
ends  of  the  coils.  The  circles  at  the  center  of  the  figure  represent 
the  collecting  rings,  one  being 
shown  inside  the  other  for  clear- 
ness. The  arrows  represent  the 
direction  of  the  current  at  a  given 
instant.  All  electromotive  forces 
induced  under  N  poles  are  in  one 
direction,  and  all  electromotive 
forces  induced  under  S  poles  are 
in  the  opposite  direction.  These 
remarks  apply  to  Figs.  143  to  150 
inclusive. 

Fig.  144  represents  a  single- 
phase  winding  distributed  in  two  slots  per  pole,  all  the  coils  being 
connected  in  series.  The  sketch  b,  is  a  sectional  view  of  a  portion 
of  the  armature  core,  showing  two  slots.  i 

Two=Phase  Windings.  The  two-phase  winding  consists  of  two 
independent  single-phase  windings  on  the  same  armature,  each  being 
connected  to  a  separate  pair  of  collecting  rings,  as  shown  in  Figs. 
145  and  146.  Fig.  145  shows  a  two-phase  concentrated  winding, 
one  slot  per  pole  for  each  phase.  Fig.  146  shows  a  two-phase  wind- 
ing distributed  in  two  slots  per  pole  for  each  phase.  In  each  of  the 
figures  (145  and  146),  the  winding  of  one  of  the  phases  is  shown 
by  dotted  lines,  to  distinguish  it  from  that  of  the  other. 


Fig.  144.      Single-Phase  Armature  Winding 
Two  Slots  Per  Pole 


A^fA 


Fig.  145.     Two-Phase  Concentrated 
Armature   Winding 


Fig.  146.     Two-Phase  Armature  Wind- 
ing, Two  Slots  Per  Pole  Per  Phase 


Three=Phase    Windings.      The    three-phase    winding  consists 
of  three  independent  single-phase  windings  on  the  same  armature, 


144 


ALTERNATING-CURRENT  MACHINERY 


the  terminals  of  the  individual  windings  being  connected  accord- 
ing to  the  Y  scheme  or  A  scheme,  as  explained  on  page  145.     Fig. 


Fig.  147.     Three-Phase  Concen- 
trated Winding  Y-Connected 


Fig.  148.     Three-Phase  Concen- 
trated Winding  A-Connected 


147  shows  a  three-phase  concentrated  winding  (one  slot  per  pole  for 
each  phase),  Y-connected.  Fig.  148  shows  the  same  winding  A- 
connected.  In  Figs.  147  and  148  the  winding  for  phase  A  is  shown 
by  heavy  full  lines;  the  winding  for  phase  B  is  shown  by  light  full 
lines;  and  the  winding  for  phase  C  is  shown  by  dotted  lines. 

The  Y  connection  gives  I/  3  times  as  much  electromotive  force 
between  collecting  rings  as  the  A  connection  for  the  same  total 
number  of  conductors  per  phase,  and  is  the  more  suitable  for  high 
electromotive  force  machines.  The  A  connection,  on  the  other 

hand,  is  especially  adapted  for 
machines  for  large  current  out- 
put. The  line  current  is  ]/  3 
times  as  great  as  the  current  in 
each  winding  in  a  A-connected 
armature. 

Fig.  149  shows  a  three-phase 
bar  winding  distributed  in  two 
slots  per  pole  for  each  phase. 
The  sketch  6,  is  a  sectional  view 
of  one  slot  containing  a  single  con- 
ductor in  the  form  of  a  rectang- 
ular bar. 

Fig.  150  shows  a  three-phase  coil  winding  distributed  in  two 
slots  per  pole  for  each  phase  and  arranged  in  two  layers,  there  being 
as  many  coils  on  the  armature  as  there  are  slots,  so  that  portions  of 


Fig.   149.     Three-Phase  Bar  Winding, 
Two  Slots  Per  Pole  Per  Phase 


ALTERNATING-CURRENT  MACHINERY 


145 


two  coils  lie  in  each  slot,  one  above  the  other.  The  portions  of  the 
coils  represented  by  full  lines  lie  in  the  upper  parts  of  the  slots,  and 
the  adjacent  dotted  portions  lie  in  the  bottoms  of  the  same  slots. 
The  sketch  b  is  a  sectional  view  of  one  slot  showing  two  half-coils 
one  above  the  other,  thus  constituting  a  two-layer  winding. 

The  method  of  connecting  up  the  separate  windings  of  a  three- 
phase  alternator  is  as  follows: 

Y  CONNECTION.  The  terminals  of  the  individual  windings  which~are 
to  be  connected  to  the  common  junction  and  to  the  collecting  rings,  may 
be  determined  as  follows:  Consider  the  instant  when  winding  A  is  squarely 
under  the  poles,  as  shown  in  Fig.  147.  The  electromotive  force  in  this  wind- 
ing (and  the  current  also,  if  the  circuit  is  non-inductive)  is  a  maximum,  and 
the  currents  in  the  other  two  phases  B  and  C  are  each  half  as  great.  If  wind- 
ing A  is  connected  so  that  its  current  is  flowing  away  from  /c,  windings  B 
and  C  must  be  connected  so  that  their  currents  flow  towards  k. 

A  CONNECTION.  The  three 
windings  form  a  closed  circuit 
when  A  connected.  The  total 
electromotive  force  around  this 
circuit  at  any  instant  must  be 
zero.  Consider  the  instant  when 
winding  A  is  squarely  under  the 
poles,  as  shown  in  Fig.  148.  The 
electromotive  force  in  this  wind- 
ing is  a  maximum,  and  the  elec- 
tromotive forces  in  the  other  two 
windings  are  each  half  as  great. 
Then  winding  A  is  connected  to 
any  pair  of  rings,  say  1  and  2; 
winding  B  is  connected  to  ring  3 
and  ring  1  (or  2);  and  winding 
C  kis  ^connected  to  ring  3  and  ring 
2  (or  1);  these  connections  are 
made  so  that  the  electromotive 
forces  at  the  given  instant  are  in  the  directions  indicated  by  the  arrows  in  Fig.  148. 


F-      150 


Three-Phase  Coil  Winding,  Two  Slots 
Per  Pole  Per  Phase 


COMMERCIAL  TYPES  OF  MACHINES 

Alternators  are  of  three  types,  differing  in  the  means  employed  for 
causing  the  conductors  to  cut  the  magnetic  flux  from  the  field  magnet, 
viz,  revolving-armature  type;  revolving-field  type;  and  inductor  type- 

Of  these  three  types  the  one  which  experience  has  proven  the 
fittest  to  survive  is  the  revolving  field  type  and  it  is  today  generally 
adopted  by  manufacturers  as  the  standard.  Alternators  of  the 
revolving  Armature  type  are  still  manufactured  to  supply  special 


146  ALTERNATING-CURRENT  MACHINERY 

demands  and  for  small  isolated  electric  plants,  and  they  may  be  found 
in  satisfactory  operation  in  many  of  the  older  and  smaller  electric 
central  stations. 

The  inductor  type  is  rarely  if  ever  built  today.  In  this  type 
both  the  field  winding  and  the  armature  (core  and  winding)  are 
stationary,  and  the  magnetic  flux  produced  by  the  field  winding  is 
caused  to  move  past  the  armature  conductors  by  means  of  a  revolving 
iron  structure  furnished  with  polar  projections  called  the  inductor. 
The  advantages  of  this  type,  viz,  absence  of  moving  wires,  collecting 
devices  and  brushes,  with  the  consequent  minimum  cost  of  attend- 
ance are  outweighed  by  the  disadvantages  of  increased  size,  weight, 
and  cost,  and  by  the  lower  efficiency  and  poorer  (higher)  regulation 
especially  when  supplying  current  to  inductive  loads  such  as  induc- 
tion motors  or  other  apparatus  having  a  power  factor  less  than 
unity.  On  account  of  these  disadvantages  the  manufacture  and  use 
of  inductor  alternators  have  been  virtually  discontinued. 

The  revolving-armature  type  of  alternator  is  limited  to  a  general 
power  and  lighting  distribution  where  only  a  moderate  voltage  is 
required.  Machines  of  this  type  are  comparatively  cheap  to  build. 
They  can  be  automatically  compounded  by  the  use  of  composite  field 
windings  without  any  compilation  of  parts,  which  is  not  the  case 
with  the  revolving  field  type.  They  can  be  furnished  with  an  aux- 
iliary armature  winding  and  small  commutator  for  exciting  their 
fields,  thus  dispensing  with  any  external  exciter. 

The  revolving-armature  type,  on  the  other  hand,  is  not  suit- 
able for  generating  either  high  or  low  voltages,  on  account  of  the 
difficulties  of  insulating  the  armature  conductors  and  collecting 
rings  in  the  first  case  (high  voltage),  and  of  collecting  a  large  arma- 
ture current  in  the  second  case  (low  voltage). 

The  advantages  of  the  revolving  field  type  over  the  revolving 
armature  type  are  as  follows : 

(1)  The  revolving  field  type  gives  more  space  for  the  armature 
winding,  and  thus  permits  the  stationary  armature  to  be  easily 
insulated  to  withstand  a  testing  pressure  of  over  30,000  volts. 

Large  alternators  with  stationary  armatures  have  been  built 
to  generate  voltages  up  to  20,000;  but  it  is  doubtful  whether,  on  the 
whole,  it  is  economical  to  build  them  for  voltages  greater  than  about 
13,000. 


ALTERNATING-CURRENT  MACHINERY  147 

(2)  The  insulation  of  the  armature  is  relieved  from  the  strains 
due  to  centrifugal  force  at  high  speeds.     Furthermore,  a  revolving 
field  can  be  made  stronger  and  more  compact  than  a  revolving  arma- 
ture and,  therefore,  the  revolving-field  type  of  alternator  is  much  the 
better  suited  to  the  high  speeds  employed  in  alternators  driven  by 
steam  turbines. 

(3)  The  number  of  collecting  rings  is  reduced  to  a  minimum, 
viz,  two,  and  the  amount  of  electrical  energy  transmitted  through 
them  is  only  about  two  per  cent  of  that  which  would  have  to  be  trans- 
mitted through  the  collector  rings  of  a  revolving  armature  alternator 
of  the  same  capacity.  The  voltage  between  the  collector  rings  is  also 
relatively  small,  being  either  125  volts,  or  250  volts  in  the  larger 
machines. 

REVOLVING=ARMATURE  ALTERNATORS 

In  this  type  of  alternator,  the  field  magnet  is  stationary,  while 
the  armature  is  mounted  on  a  shaft  and  is  driven  (by  means  of  a  belt 
or  mechanical  coupling)  by  the  prime  mover,  which  may  be  either 
a  steam  engine,  a  steam  turbine,  or  a  water  wheel.  The  current 
induced  in  the  armature  conductors  is  delivered  to  the  external 
circuit  through  collector  rings  on  which  brushes  rub.  The  armature 
may  be  wound  for  a  single-phase  current  (having  two  collector  rings), 
for  two-phase  currents  (having  four  collector  rings),  for  three-phase 
currents  (having  three  collector  rings),  etc.  The  revolving-armature 
type  is  used  almost  exclusively  for  alternators  of  small  output  and 
moderate  voltage. 

Although  single-phase  alternators,  and  especially  those  of  the 
revolving-armature  type  have  been  virtually  superseded  by  poly- 
phase machines  of  the  revolving-field  type,  still  there  are  today  a 
number  of  the  older  fashioned  alternators  in  regular  use  in  some  of 
the  smaller  electric  lighting  stations.  For  this  reason  some  of  the 
typical  features  of  these  machines  will  be  described  before  considering 
the  more  modern  designs. 

Fort  Wayne  Single=Phase.  Fig.  151  shows  a  90-kilowatt  1,100- 
volt  8-pole  single-phase  belt-driven  alternator  manufactured  by  the 
Fort  Wayne  Electric  Company,  of  Fort  Wayne,  Indiana.  It  is 
designed  to  be  driven  at  a  speed  of  900  revolutions  per  minute;  hence 


the  frequency  of  its  electromotive  force  is  -  —  ~  —  =  60  cycles  per  sec- 


148 


ALTERNATING-CURRENT  MACHINERY 


ond.    The  figure  shows  the  two  collector  rings  adjacent  to  the  arma- 
ture, also  the  rectifying  commutator  with  its  brushes  for  supplying 


s 


uni-directional  current  to  the  coarse  wire  coils  of  the   composite 
field  winding.* 


ALTERNATING-CURRENT  MACHINERY  149 

The  exciter  is  a  shunt-wound  4-pole  2-kilowatt  direct-current 
generator  running  at  a  speed  of  1,400  revolutions  per  minute.  It 
is  shown  belted  to  a  pulley  on  the  alternator  shaft.  The  current 
from  this  exciter  is  led  to  the  fine  wire  coils  of  the  composite  field 
winding.  Each  field  pole  of  the  alternator  is  provided  with  two 
coils  wound  on  one  and  the  same  spool — one  of  coarse  wire,  sup- 
plied with  current  from  the  rectifying  commutator;  and  the  other 
of  fine  wire,  supplied  with  current  from  the  exciter. 

The  field  structure,  base-plate,  and  pedestals  are  cast  in  one 
piece,  and  the  whole  machine  rests  upon  a  cast-iron  sub-base.  This 
sub-base  is  provided  with  slide  rails  along  which  the  machine  may 
be^moved  by  means  of  a  screw  turned  by  a  ratchet  and  lever,  as 
shown — this  for  the  purpose  of  adjusting  the  tension  of  the  main 
driving  belt.  The  field  poles  are  "built  up"  of  sheet-iron  stamp- 
ings and  are  held  together  by  long  bolts.  These  field  poles  are  ar- 
ranged in  the  mould  in  which  the  field  frame  is  cast,  and  are  thus 
cast-welded  to  the  frame. 
The  field  coils  are  ma- 
chine wound  on  spools, 
with  insulated  copper 
wire,  the  coarse  wire  coils 
being  wound  on  top  of  the 

•fin^  \\nre*  nnil«       TVif*  fiplfl    Fig.  152.     German  Silver  Resistance  Coils  in  Shunt  with 
US.  le/ltfi  Coarse  Wire  Field  Coils 

spools  are  held  in  position 

on  the  field  poles  by  brass  collars  fixed  to  the  outer  ends  of  the  poles. 

The  coarse  wire  coils,  connected  in  series,  are  supplied  with 
uni-directional  current  from  the  rectifying  commutator ;  and  the  entire 
set  is  shunted  with  resistance  consisting  of  a  strip  of  German  silver 
wound  on  an  insulated  form,  Fig.  152,  and  mounted  on  an  insulated 
block  inside  the  hollow  pedestal  on  the  collector  end  of  the  machine. 

The  armature  is  of  the  iron-clad  ring  type  built  up  of  small 
overlapping  sheet-steel  punchings,  annealed  and  japanned  to  reduce 
the  loss  caused  by  hysteresis  and  eddy  currents. 

The  armature  winding  is  of  the  concentrated  or  uni-coil  type. 
The  coils  are  wound  by  hand  directly  on  each  armature  tooth, 
insulated  copper  ribbon  being  used.  Armature  coils  are  generally 
wound  on  formers,  and  afterwards  sprung  into  place  in  the  slots 
on  the  armature  core. 


150 


ALTERNATING-CURRENT  MACHINERY 


Westinghouse  Uni=Coil  Armature.  Figs.  153  and  154  show  a 
Westinghouse  single-phase  uni-coil  armature  very  much  like  the 
one  used  in  the  alternator  shown  in  Fig.  151.  The  coils  of  the  West- 


Fig.  153.     Westinghouse  Single-Phase  Uni-Coil  Armature  Core 

inghouse  alternator,  however,  are  machine- wound  on  formers;  and, 
after  being  taped,  varnished,  and  baked,  they  are  spread  out  slightly 
so  as  to  pass  over  the  teeth,  and  are  forced  into  place  in  the  deep 
slots,  being  securely  held  there  by  wooden  wedges.  Fig.  153 
shows  the  core  unwound,  and  Fig.  154  the  method  of  placing 
coils  in  this  type  of  machine.  The  armature  teeth  are  T  shaped, 
and  partially  overhang  the  armature  coils,  thus  protecting  the  coils 
from  injury.  There  are,  of  course,  the  same  number  of  armature 
teeth  as  field  poles  in  the  uni-coil  armature.  Thus  Figs.  144  and  145 
show  an  armature  core  for  an  eight-pole  field. 

Fig.  155  shows  the  completed  armature  of  which  the  core  and 
coils  are  shown  in  Figs.  153  and  154.  At  the  ends  of  the  armature 
are  two  brass  shields  for  protecting  the  ends  of  the  armature  coils. 


Fig.  154.     Westinghouse  Single-Phase  Armature  with  Coils  in  Place 

The  ring-oiling,  spherical-seated,  self -aligning  bearings  are  shown 
on  the  shaft  in  Fig.  155;  and  at  the  end  of  the  shaft  are  shown  the  two 


ALTERNATING-CURRENT  MACHINERY  151 

collecting  rings  and  the  rectifying  commutator.  The  wires  leading 
to  the  collector  rings  and  to  the  rectifying  commutator  are  led  through 
the  shaft,  which  is  made  hollow. 


Fig.  155.      Westinghouse  Single-Phase  Uni-Coil  Armature  Complete 


Fig.  156.     Westinghouse  Single-Phase  Armature  Core  and  Coils.     Distributed  Winding 


Fig.  157.     Westinghouse  Single-Phase  Distributed  Winding  Armature  Complete 

Westinghouse  Armature  with  Distributed  Winding.  Figs.  156 
and  157  show  a  single-phase  Westinghouse  alternator  armature  with 
distributed  winding.  Three  armature  coils  are  shown  in  Fig.  156 


152 


ALTERNATING-CURRENT  MACHINERY 


separately  and  also  grouped  together.  The  manner  of  grouping 
shown  is  carried  out  when  the  armature  coils  are  assembled  on  the 
armature  core;  and  there  are  as  many  of  these  groups  of  armature  coils 
as  there  are  poles  in  the  field.  Fig.  157  shows  the  finished  armature 
with  end  shields,  collecting  rings,  and  rectifying  commutator. 

In  assembling  the  stampings  of  an  armature  core  a  stiff,  cor- 
rugated stamping  (or  its  equivalent)  is  inserted  at  intervals  between 
groups  of  the  flat  sheet-iron  stampings,  thus  leaving  radial  air  ducts 
from  the  inside  to  the  outside  of  the  armature  core.  These  are  known 
as  ventilating  ducts,  and  their  object  is  to  permit  of  a  free  circulation  of 

cool  air  through  the  arma- 
ture core  and  coils,  thus 
preventing  excessive  rise  of 
temperature.  The  motion 
of  the  armature  causes  air 
to  be  drawn  in  through  the 
end  shields  to  the  interior 
of  the  armature,  whence  it 
is  thrown  out  through  the 
radial  ducts,  as  in  the  case 
of  a  ventilating  fan.  Four 
of  these  spaces  between 
stampings  can  be  seen  in 
Figs.  156  and  157;  and  the 
armature  "spider"  and  end 
shields  at  each  end,  as 
shown,  are  provided  with  apertures  for  admitting  cool  air. 

Field  Structure  for  Westinghouse  180=Kw.  Alternator.  Fig. 
158  shows  the  field  structure  of  a  Westinghouse  180-kilowatt  alter- 
nator before  the  field  coils  have  been  placed  upon  it.  The  field  poles 
(laminated)  are  shown  projecting  radially  inwards  from  the  cast- 
iron,  ring-shaped  yoke.  The  yoke  is  cast  in  two  pieces,  and  the  upper 
half  is  bolted  to  the  lower  half,  as  shown  in  the  figure.  The  yoke  is 
divided  in  this  way  in  order  that  the  upper  half  may  be  unbolted  and 
may  be  lifted  off  by  means  of  the  eye-bolt  on  the  top  of  the  machine, 
thus  giving  easy  access  to  the  armature  for  inspection  and  repairs. 

General  Electric  Three=Phase  Alternator.  Fig.  159  shows  a  view 
of  a  25-kilowatt  three-phase  revolving-armature  type  of  alternator 


Fig.  158.     Field  Structure  of  a  Westinghouse 
180-Kw.  Alternator 


ALTERNATING-CURRENT  MACHINERY 


153 


built  by  the  General  Electric  Company  for  use  in  small  isolated 
plants.  The  alternator  is  belt-driven  at  1,800  r.  p.  m.  and,  there- 
fore, requires  four  poles  to  give  a  frequency  of  60  cycles. 

The  view  shows  three  collector  rings  mounted  on  the  armature 
shaft  for  the  collection  of  the  three-phase  currents,  and  a  com- 
mutator between  the  rings  and  the  armature  core.  The  special 
feature  of  this  machine  is  that  it  requires  no  separate  exciter,  for  the 
armature  is  wound  with  two  distinct  windings,  one  of  which  is  con- 


Fig.  159.     General   Electric   25-Kw.  Three-Phase  Alternator 

nected  to  the  commutator  and  supplies  the  exciting  current  to  the 
field  coils.  The  main  armature  winding  generates  the  three-phase 
alternating  currents  and  is  connected  to  the  three  collector  rings. 
The  field  structure  consists  of  laminated  pole  pieces  cast  into  a  rigid 
stationary  frame.  The  field-magnet  cores  are  wound  with  coils 
which  furnish  a  magnetic  flux  which  is  common  to  both  alternator 
and  exciter  armature  windings.  These  machines  are  built  in  three 
sizes  and  rated  at  7.5,  15,  and  25  kw.  at  unity  power  factor,  and  at 


154  ALTERNATING-CURRENT  MACHINERY 

6,  12,  and  20  kw.,  respectively,  at  0.8  power  factor.  Standard 
voltages  are  120,  240,  480,  and  600  volts  at  60  cycles,  and  their  ratings 
are  based  on  either  two-  or  three-phase  operation.  For  single-phase 
operation  any  one  of  the  phases  may  be  used,  but  the  rating  then  is 
only  70  per  cent  of  the  polyphase  rating  on  account  of  the  heating. 

REVOLVING=FIELD   ALTERNATORS 

In  this  type  of  alternator,  the  armature  is  stationary,  and  the 
armature  windings  are  arranged  in  slots  on  the  inner  face  of  the 
armature  structure,  the  latter  forming  a  closed  ring  inside  of  which 
the  multipolar  field  magnet  revolves.  The  armature  structure  con- 
sists of  an  external  frame  of  cast  iron  or  steel  supported  on  a  bed- 
plate. The  armature  core  proper  consists  of  a  ring  built  up  of 
relatively  small  stampings  of  sheet  iron  dovetailed  into  the  external 
frame  and  pressed  together  between  two  flanges  by  bolts.  The  ex- 
ternal frame  in  this  type  of  alternator  does  not  to  any  perceptible 
extent  carry  lines  of  force,  that  is,  it  does  not  form  a  part  of  the 
magnetic  circuit,  but  serves  only  as  a  support  for  the  laminated 
armature  core. 

Construction.  Frame.  When  a  given  type  of  alternator  has 
been  standardized  by  a  manufacturer,  it  is  customary  to  lay  out  a 
complete  line  of  machines,  which  differ  in  weight  by  fifteen  or  twenty 
per  cent  between  consecutive  sizes.  The  capacity  of  a  given  frame 
is  dependent  upon  speed,  voltage,  and  specified  performances. 

Frames  are  made  in  two  general  styles,  one  called  the  box  type 
and  the  other  the  skeleton  type.  The  box  type  consists  of  a  single 
casting  for  the  smaller  sizes  until  an  outside  diameter  of  about  8 
feet  is  reached.  Above  this  diameter  the  frame  castings  are  usually 
divided  into  upper  and  lower  sections,  split  construction  being  neces- 
sary on  account  of  the  limitation  imposed  in  handling  and  shipping. 

The  skeleton  type  consists  of  two  side  castings  between  which 
substantial  spacing  rods  are  set  at  suitable  intervals.  For  manu- 
facturing reasons  the  skeleton  type  is  preferred  for  certain  sizes  of 
machines,  as  its  construction  readily  permits  of  changes  being  made 
in  the  assembling  of  the  armature-core  laminations  without  neces- 
sitating a  change  in  the  patterns. 

The  main  point  to  be  looked  after  in  alternator  frames  is  a 
design  that  will  give  the  maximum  of  rigidity.  The  only  function 


ALTERNATING-CURRENT  MACHINERY 


155 


of  the  frames  of  rotating-field  alternators  isxto<hold*rigidly*  in  place 
the  parts  composing  the  stationary  element.  The  frame  is  further 
designed  with  openings  at  the  back  of  the  laminations  to  give  thorough 
ventilation.  The  frames  contain  dovetailed  slots  into  which  the 
laminated  iron  for  the  stationary  part  is  pressed.  The  laminated 


Fig.  160.     AIHs-Chalmers  Revolving  Field  Alternator  with  Rotor  Removed 

iron,  however,  carries  all  the  magnetic  flux  and  the  frame  is  simply 
for  the  purpose  of  holding  it  in  position. 

Armature.  The  armature  (stator)  core  consists  of  sheet-iron 
laminations  carefully  selected  and  annealed  before  assembling  in 
order  to  reduce  core  loss.  The  punchings  are  stacked  together  and 
held  rigidly  in  place  by  heavy  steel  clamping  fingers,  the  outer  cir- 
cumference of  the  laminations  being  dovetailed  for  fastening  to  the 


156  ALTERNATING-CURRENT  MACHINERY 

frame  and  thejinner  circumference  being  slotted  to  receive  the  armature 
windings.  The  punchings  are  separated  at  intervals  by  ventilating 
plates,  which  give  opportunity  for  air  to  circulate  freely  through  the 
ducts  thereby  created  and  out  at  the  open  back  of  the  frame.  Heavy 
end  plates  are  supplied  at  both  ends  of  the  laminations  so  as  to  pre- 
vent their  bulging  out  at  the  ends. 


Fig.  161.     Allis-Chalmers  Revolving  Field  Alternator  with  One  Bearing  Removed  and 

Rotor  in   Foreground 

Bed  Plate  and  Bearing  Pedestals.  Alternators  may  be  divided 
into  three  types  depending  upon  the  method  of  driving,  viz,  engine 
type,  coupled  type,  and  belted  type.  In  the  engine  type  the  revolving 
element  is  mounted  directly  on  an  extended  engine  shaft.  In  the 
coupled  type,  as  the  name  indicates,  the  alternator  is  entirely  self- 
contained  and  is  designed  to  be  coupled  directly  to  a  prime  mover, 
usually  a  water  wheel.  In  the  belted  type  the  alternator  is  self- 


ALTERNATING-CURRENT  MACHINERY 


157 


contained  and  mounted  entirely  separate  from  the  prime  mover  and 
connected  therewith  by  a  belt. 

The  almost  universal  practice  is  to  make  the  bed  plate,  the  in- 
dividual bearing  pedestals  and  the  frame  all  separate.  The  parts  are 
properly  machined  and  bolted  together.  This  practice  enables  the 
manufacturer  to  use  the  same  parts  for  the  various  types  of  machines, 
and  to  vary  the  combinations  of  bed  plate,  bearing  pedestal,  and 
frames  to  suit  any  case  that  may  arise.  For  some  of  the  smaller 
belted  machines  a  bracket  bear- 
ing support  instead  of  pedestal 
on  a  bed  plate  is  used.  This  al- 
lows the  size  of  the  bed  plate  to 
be  reduced  to  a  minimum. 

Fig.  160  shows  a  frame  of 
the  box  type  with  the  lami- 
nations and  windings  placed 
therein  complete.  Fig.  161 
shows  the  same  alternator  with 
one  of  the  end  brackets  con- 
taining a  bearing  removed  and 
the  rotor  (revolving  field)  in  the 
foreground.  This  alternator  is 
for  belt  driving  and  is  called 
"TypeAB"  by  the  Allis-Chal- 
mers  Company  who  manufac- 
ture it. 

Fig.  162  is  a  section  of  an 
alternator  showing  the  method 

of  dovetailing  the  core  laminations  to  the  stator  frame.  Heavy  clamp- 
ing rings  or  end  plates  are  mounted  on  both  sides  of  the  core  by  means 
of  bolts,  and  supporting  fingers  extend  along  the  teeth  on  either  side 
of  the  slots,  as  shown  in  Figs.  163  and  164. 

An  ample  circulation  of  air  for  cooling  is  provided  by  means  of 
ducts  formed  by  suitable  spacing  blocks  inserted  at  intervals  be- 
tween the  laminations,  as  may  be  seen  in  Figs.  163  and  165. 

Armature  Coils.  The  armature  coils  are  wound  on  formers  and, 
the  slots  being  open,  the  coils  can  be  easily  removed  and  replaced 
in  case  of  injury.  They  are  taped  and  impregnated  with  an  insulat- 


Fig.   162.     Portion  of  General  Electric  Alternator 

Showing  Method  of  Dovetailing  Core 

Laminations 


158 


ALTERNATING-CURRENT  MACHINERY 


ing  compound.  After  being  tested,  the  coils  are  inserted  in  the 
armature  slots  which  are  lined  with  horn  fiber  and  retaining  wedges 
of  wood  are  dovetailed  into  V-shaped  notches  on  either  side  of  the 
slot,  as  shown  in  Figs.  163  and  165. 

Supporting  Ring.  Where  heavy  windings  project  beyond  the 
laminations,  an  additional  support  is  provided  by  means  of  an  in- 
sulated metal  ring,  to  which  the  outer  ends  of  the  coils  are  fastened; 
the  coils  are  thereby  protected  from  mechanical  displacement,  or 
distortion  due  to  the  magnetic  disturbances  caused  by  violent  fluctua- 


Fig.  163.     Armature  Coil  Support  Construction — General  Electric  Alternator 

tions  of  the  load  or  short-circuits.  Fig.  165  shows  a  section  of  a  sup- 
porting ring  of  this  type  and  indicates  the  method  of  connecting  the 
coils  to  it.  In  order  to  admit  of  the  prompt  replacement  of  damaged 
coils,  sufficient  space  is  usually  provided  between  the  alternator  bear- 
ings to  allow  ample  movement  of  the  stator  to  permit  of  ready  access  to 
both  stator  and  rotor  coils.  Where  space  economy  necessitates  the  use 
of  a  short  shaft,  access  to  the  windings  may  be  procured  by  disconnect- 
ing some  of  the  coils  and  lifting  the  upper  half  of  the  stator. 


ALTERNATING-CURRENT  MACHINERY 


159 


Terminals.  Flexible  leads  are  brought  through  the  frame  of 
the  machine  near  the  bottom  and  connected  directly  to  the  line  or  to 
terminal  blocks  which  are  mounted  on  the  frame.  The  former 
arrangement  is  usually  employed. 

Rotating  Field.  The  construction  of  the  rotating  field  is  very 
similar  to  that  of  the  stationary  armature  except  that  the  punchings 
are  dovetailed  into  a  rotating  spider  instead  of  a  stationary  frame. 

Fig.  166  is  a  view  of  a  laminated  field  core  in  process  of  construc- 
tion, showing  how  the  laminations  are  assembled  with  overlapping 


Fig.  164.  Armature  Coil  Support  Construction — General  Electric 
Alternator 

joints,  and  are  fastened  to  the  supporting  structure.   Three  individual 
stampings  are  shown  separate  in  the  figure. 

In  assembling  the  stampings  that  form  the  built-up  structure 
of  field  poles  and  yokes,  the  stampings  break  joints  in  order  to  enable 
the  structure  to  resist  centrifugal  force  without  bringing  undue  stress 
upon  the  central  supporting  structure.  The  stampings  are  per- 
forated with  a  number  of  holes,  as  shown.  These  holes  register  in 
the  built-up  structure,  and  bolts  are  passed  through  them  in  order 


160  ALTERNATING-CURRENT  MACHINERY 


Fig.   165.     Portion  of  General  Electric  Stationary  Armature 
Showing  Method  of  Connecting  Armature  Coils 


Fig.  166.     Method  of  Constructing  Laminated  Field  Cores 


ALTERNATING-CURRENT  MACHINERY 


161 


167. 


Field  Spider  Showing  Laminated 
Construction 


to  clamp  the  laminations  together.  At  intervals  of  about  3  inches,  the 
laminations  are  separated  by  a  corrugated  lamination  (or  its  equiv- 
alent), which  serves  to  form  ventilating  ducts  that  extend  in- 
wardly to  large  openings  in  the 
cast-iron  segments.  These  ven- 
tilating ducts  in  the  revolving 
field  register  or  coincide  with 
corresponding  ducts  in  the  ex- 
ternal stationary  armature. 

The  field-pole  tips  are 
beveled  so  as  to  produce  a 
distribution  of  magnetic  flux 
which  will  give  approximately 
a  sine  wave  electromotive 
force  at  no  load  (under  load 
conditions  the  wave  would  be 
somewhat  distorted). 

For  alternators  of  rela- 
tively small  rated  output,  from 
about  30  to  200  kilowatts,  and  designed  for  belt  driving  at  a  high 
speed,  another  type  of  construction  is  used  by  the  Westinghouse 
Company  in  their  "Type  G"  generators. 

The  central  portion  of  there- 
volving  part,  shown  in  Fig.  167, 
is  a  laminated  spider,  built  up  of 
thin  steel  plates  assembled  upon 
a  mandrel  and  firmly  riveted  to- 
gether under  hydraulic  power. 

The  core  is  accurately 
bored  and  the  spider  is  pressed 
upon  the  shaft  in  the  same  man- 
ner as  a  cast-steel  spider.  The 
poles,  one  of  which  is  shown  in 
Fig.  168,  are  also  built  up  of 
steel  laminations  of  the  same 
thickness  as  those  of  the  spider,  and  riveted  together.  Each  pole  is 
dovetailed  into  the  spider  and  retained  by  two  taper  steel  keys.  Fig. 
169  shows  the  rotor  core  with  pole  pieces  assembled  on  the  spider. 


Fig.   168.     One  of  the  Field  Poles  Which  Fits 
into  the  Core  of  Fig.  167 


162 


ALTERNATING-CURRENT  MACHINERY 


The  pole  pieces  are  thereby  securely  held  in  place  during  opera- 
tion; but  poles  and  coils  may  be  easily  taken  out  when  desired,  by 

knocking  out  the  steel  keys. 
The  field  coils  of  these 
"type  G"  alternators  are 
wound  with  wire,  and  the 
complete  rotor  or  revolving 
field  for  the  30-  and50-kilo- 
watt  machines  is  shown  in 
Fig.  170.  The  pole  pieces 
of  this  type  of  alternator 
rated  above  50  kilowatts  up 
to  200  kilowatts  are  pro- 
vided with  practically  closed 
slots  in  the  pole  face  for  the 
copper  bars  of  a  "squirrel- 
cage"  winding.  The  slots  are 
plainly  shown  in  Fig.  167, 
and  the  rotor  with  the  cage 
winding  in  place  is  shown 
in  Fig.  171.  This  winding, 
also  called  an  "amortisseur" 
winding,  acts  as  an  effective 
damper  to  prevent  hunting 
between  machines  operated 
in  parallel.  The  collector 
rings  for  conveying  through 
brushes  the  direct  current 
for  exciting  the  field  coils, 
shown  to  the  right  in  Fig. 
171,  are  of  cast  iron,  insu- 
lated from  the  iron  bushing 
over  which  they  are  shrunk 
by  V-shaped  mica.  Another 
rotor  construction  is  often 
employed  in  the  case  of 
large  alternators  designed  for  engine  driving.  In  such  machines  the 
revolving  field  structure  consists  of  laminated  pole  pieces  bolted 


Fig.  169. 


Rotor  Core  with  Pole  Pieces  Assembled 
on  Spider 


Fig.  170.     Rotor  of  Fig.  169  with  Field  Coils  in  Place 


ALTERNATING-CURRENT  MACHINERY  1C3 

to  a  cast-steel  or  iron  ring,  which  is  connected  to  the  hub  by  arms  of 
suitable  section,  as  shown  in  Fig.  172.  The  pole  pieces  are  built 
of  sheet  iron,  spreading  at  the  pole  face  so  as  to  secure  not  only  a 
large  sectional  area  for  the  magnetic  flux  passing  from  the  poles 
into  the  steel  ring,  but  also  to  hold  the  field  coils  in  place. 

In  Fig.  172  is  shown  the  revolving  field  of  an  alternator  built  by 
the  General  Electric  Company  for  direct  connection  to  reciprocating 
engines.  In  Fig.  173  is  shown  the  laminated  pole  piece  and  one  of  the 
strip-wound  field  coils,  and  in  Fig.  174  is  shown  a  complete  pole  piece 
with  its  coil  ready  for  bolting  to  the  face  of  the  supporting  structure. 

Field  Coils.  The  field  coils  are  wound  on  spools  or  in  moulds. 
Wire  is  used  for  the  smaller  generators  and  in  those  of  larger  capacity 
a  single  strip  of  flat  copper  bent  on  edge  is  usually  employed,  as 
shown  in  Fig.  173,  so  that  the  edge  of  every  turn  is  exposed  to  the 


Fig.   171.     Rotor  with  Squirrel-Cage  Winding 

air  to  facilitate  radiation,  and  hence  cooling.  The  strap-wound  coils 
are  insulated  between  turns  with  asbestos  strip  forming  a  fireproof 
coil  which  is  practically  indestructible.  The  wire  coils  are  wound 
dry  and  treated  with  insulating  varnishes.  In  every  case  the  insula- 
tion provided  is  easily  sufficient  for  the  low  voltage  to  which  the  field 
winding  is  subjected. 

Excitation.  The  direct  current  for  exciting  the  field  magnet 
is  carried  in  to  the  revolving-field  structure -by  means  of  brushes 
rubbing  on  collector  rings  to  which  are  connected  the  terminals  of 
the  field  coils.  The  latter  are  all  connected  in  series. 


164 


ALTERNATING-CURRENT  MACHINERY 


Alternators  are  usually  separately  excited  by  125-volt  direct 
current.     This  voltage  is  generally  regarded  as  standard,  is  easily 


Fig.   172. 


Rotor  Construction  with  Laminated  Pole  Pieces 
Bolted  to  Cast-Iron  or  Steel  Ring 


handled,  and  lends  itself  readily  to  the  operation  of  lights  and  small 
auxiliary  machines  in  power  stations.    For  large  alternators  a  voltage 


Fig.  173. 


Laminated  Pole-Piece  and  Strip-Wound  Field 
Coil  for  Rotor  of  Fig.  172 


of  250  has  frequently  been  employed  and  field  coils  can  be  wound 
for  this  voltage  when  required.     The  lower    voltage  is  generally 


ALTERNATING-CURRENT  MACHINERY 


165 


to  be  preferred,  principally  because  it  permits  the  use  of  strap-wound 
field  coils.  For  generators  large  enough  to  employ  field  coils  of  this 
type  at  the  higher  voltage,  250-volt  excitation  is  not  objectionable. 
The  limitation  is  based  on  the  fact  that  very  thin  copper  strap  cannot 
be  successfully  bent  edgewise. 

Water=Wheel=Driven  Alternators.  Alternating-current  gen- 
erators which  are  designed  to  be  directly  connected  to  water-wheels 
are  usually  driven  at  speeds  of  from  about  150  to  600  revolutions 
per  minute,  depending  upon  the  size  and  type  of  water  wheel  used. 
The  customary  range  of  speeds  in  water-wheel-driven  alternators  is, 
therefore,  intermediate  between  the  slow-speed  range  of  the  engine- 
driven  type  and  the  high-speed  range  of  the  steam  turbine-driven 
type.  Alternators  of  the  water-wheel  type  are  built  both  with  hori- 
zontal and  with  vertical  shafts  according  to  conditions.  The  form 
of  foundation  used  varies  with  the  service  for  which  they  are  intended. 
For  the  machines  with 
horizontal  shafts,  cast-iron 
or  channel  iron  bases  are 
used,  and  in  some  cases 
simple  foundation  plates 
are  provided  for  the  stator 
with  separate  sole  plates 
for  the  bearing  standard. 
For  vertical  shaft  alterna- 
tors the  base  is  construct- 
ed either  for  mounting 
directly  on  the  turbine 
casing  or  on  separate 
foundations  above  the 
wheel  pit.  The  shafts  are 
keyed  to  the  rotor  and  are  arranged  for  coupling  to  the  water- 
wheel  shaft. 

The  style  of  bearings  adopted  depends  upon  the  size  of  the 
alternator.  Some  of  the  smaller  machines  are  furnished  with  end 
shield  bearings  but  the  standard  form  for  horizontal  shaft  alternators 
is  a  pedestal  bearing  arranged  for  oil-ring  lubrication.  Large  machines 
are  often  provided  with  water-cooled  bearings  which  consist  of  a 
number  of  short  tubes  extending  horizontally  through  the  oil  well 


Fig.  174.     Completed  Field  Coil  for  Rotor  of  Fig.  172 


166  ALTERNATING-CURRENT  MACHINERY 

below  the  bearing  through  which  the  cooling  water  is  conducted, 
thereby  reducing  the  temperature  of  the  oil. 

Vertical  shaft  alternators  are  arranged  for  direct  connection  to 
the  water-wheel  shaft  and  are  usually  provided  with  one  or  two 
guide  bearings.  Step  or  suspension  bearings  arranged  for  forced  oil 
circulation  are  also  standard.  The  standard  water-wheel  type  of 
alternator  is  wound  for  three  phases,  but  it  can  be  adapted  for  two 
phases  without  change,  except  in  the  armature  coils  and  punchings. 
Where  single-phase  operation  is  required,  the  three-phase  winding 


Fig.   175.     View  of   General  Electric  Water-Wheel-Driven  Alternators 

is  furnished,  the  single-phase  current  being  taken  off  from  any  two 
of  the  three  armature  terminals.  When  thus  used  the  alternator 
full-load  rating  at  unity  power  factor  is  only  about  70  per  cent  of  the 
full-load  three-phase  rating. 

Where  transformers  are  used  to  step-up  the  voltage  for  long- 
distance power  transmission,  it  is  considered  good  practice  to  install 
alternators  wound  for  the  standard  voltages  of  2,300  or  for  6,600 
volts,  but  where  current  is  to  be  transmitted  at  the  generator  voltage, 
armature  windings  for  11,000  volts,  60  cycles,  or  for  13,200  volts, 
25  cycles,  are  considered  standard. 


ALTERNATING-CURRENT  MACHINERY 


167 


Fig.  175  is  a  general  view  of  three  water-wheel-driven  alternators 
manufactured  by  the  General  Electric  Company  and  installed  in  a 
hydroelectric  power  plant  at  Spokane,  Washington.  The  alternators 
are  each  rated  at  2,250  kilowatts,  60  cycles,  2,300  volts,  138  revolu- 
tions per  minute  and  are  wound  with  three  phases.  Each  machine 


has 


60X2X60 

-    —  —  - 

loo 


=  52  poles.     The  view  shows  the  exciters  directly 


mounted  on  an  extension  of  the  main  shaft,  and  supported  on  a 
bracket  which  is  bolted  to  the  iron  base. 

These  alternators  may  be  furnished  either  with  or  without 
direct-connected  exciters.  When  arranged  for  direct  connection, 
the  armature  of  the  exciter  is  carried  on  the  generator  shaft.  In 


Fig.   176. 


General  Electric  Alternator  with  Direct-Connected  Exciter  Bracketed  to  the 
Same  Base 


the  smaller  sizes  the  magnet  frame  is  bolted  to  the  bearing  bracket, 
as  shown  in  Fig.  176,  but  in  the  larger  sizes  special  construction  is 
used,  depending  upon  the  conditions  of  the  particular  installation. 
Fig.  176  also  shows  clearly  the  coupling  for  direct  connection  to  the 
water  wheel  at  the  right,  and  the  box  type  of  frame  with  ventilating 
apertures.  This  General  Electric  alternator  is  rated  at  3,000  kilo- 
watts, 2,300  volts,  and  514  revolutions  per  minute. 

Fig.  177  is  a  general  interior  view  of  the  new  hydroelectric 
plant  of  the  Connecticut  River  Power  Company  at  Vernon,  Vermont. 
Five  vertical  shaft  water-wheel-driven  alternators  are  used,  each 


168 


ALTERNATING-CURRENT  MACHINERY 


being  rated  at  2,500  kilowatts  2,300  volts,  60  cycles,  and  133  revolu- 
tions per  minute.  The  two  small  vertical  shaft  generators,  located 
between  the  first  and  second  large  units,  are  exciters,  each  driven 
independently  by  a  small  water  wheel  installed  below  the  main  floor 
level.  This  practice  of  using  exciters  driven  independently  of  the 
main  water  wheels  is  to  be  recommended,  as  it  is  a  safeguard  against 
a  general  shutting  down  of  the  plant  due  to  accident  to  one  or  more 
of  the  main  machines. 


Fig.  177.     Interior  View  of  Connecticut  River  Power  Company's  Hydroelectric  Plant 

Fig.  178  shows  an  armature  structure  for  a  600-kilowatt  Allis- 
Chalmers  "water-wheel  type"  alternator,  with  a  three-phase  wind- 
ing in  place.  This  winding  is  distributed  in  two  slots  per  pole  per 
phase.  The  coils  that  constitute  one  of  the  phases  are  those  of  which 
the  ends  show  most  distinctly.  The  sides  of  the  coils  belonging  to  the 
other  two  phases  lie  in  the  remaining  slots,  four  of  which  are  sur- 
rounded by  each  pair  of  coils  belonging  to  the  first  phase.  The 
manner  of  connecting  the  coils  belonging  to  each  phase  is  explained 
on  page  145.  This  armature  structure,  Fig.  178,  has  two  extensions 
cast  as  part  of  the  frame,  which  are  intended  to  rest  upon  a  foun- 


ALTERNATING-CURRENT  MACHINERY 


169 


elation  on  a  level  with  the  floor.  The  lower  part  of  the  ring  is  de- 
signed to  extend  below  the  level  of  the  floor  into  a  pit.  In  the  same 
figure  is  shown  also  a  metal  shield  which  is  screwed  to  the  frame  and 
serves  to  protect  the  ends  of  the  armature  coils. 


Fig.  178.     Stationary  Armature  for  600-Kw.  Allis-Chalmers  "Water-Wheel  Type" Alternator 

Steam  Turbine=Driven  Alternators.  This  type  of  machine, 
often  called  a  turbo-alternator,  has  been  in  the  past  few  years  greatly 
improved  and  developed.  The  steam-turbine  has  already  largely 
supplanted  the  reciprocating  engine  in  all  large  steam-electric  stations 
and  the  present  general  tendency  is  away  from  the  low-speed  engine- 


170  ALTERNATING-CURRENT  MACHINERY 

TABLE  V 
Turbine  Speeds  for  Alternators 


POLES 

2 

4 

6 

8 

10 

60  cycles 

3600 

1800 

1200 

900 

720 

25  cycles 

1500 

750 

driven  alternators  and  towards  the  high-speed  turbine-driven  alter- 
nator. The  steam  turbine  operates  most  efficiently  at  high  speeds, 
so  in  order  to  accommodate  the  generators  to  these  conditions  with 
the  standard  frequencies  of  25  to  60  cycles,  the  number  of  field  poles 
must  be  reduced  to  a  minimum.  For  this  reason  60-cycle  generators 
have  been  built  with  two  poles  for  3,600  revolutions  per  minute  in 
capacities  up  to  2,500  kilowatts,  and  with  four  poles  for  1,800  revolu- 
tions per  minute  up  to  10,000  kilowatts. 

25-cycle  alternators  are  necessarily  limited  to  a  maximum  speed 
of  1,500  revolutions  per  minute,  even  with  only  two  poles.  Two- 
pole  alternators  have  been  built  in  sizes  up  to  7,500  kilowatts,  and 
even  larger  machines  are  possible.  Table  V  gives  the  revolutions 
per  minute  which  have  been  used  for  turbo-alternators. 

The  steam  turbine  is  an  essentially  high-speed  machine  and 
the  alternator  designed  to  be  driven  by  it  must  be  constructed  with 
special  attention  to  the  effects  of  high  speed.  This  requirement 
has  developed  many  interesting  though  difficult  problems  in  design, 
such  as  constructions  able  to  withstand  the  enormous  centrifugal 
forces  developed  in  the  rotor,  amounting  to  4,000  pounds  on  each 
pound  of  material  near  the  surface  of  the  rotor.  The  important 
matters  of  securing  perfect  balance  and  freedom  from  vibration  at 
high  speeds,  of  adequate  ventilation,  and  lubrication  of  bearings,  have 
been  successfully  met. 

Advantages.  The  most  important  advantages  of  steam  turbines 
over  reciprocating  steam  engines  are: 

(1)  High  economy  at  all  loads. 

(2)  Economy  in  floor  space  and  building  material. 

(3)  Moderate  initial  cost  and  low  maintenance  expense. 

(4)  Simplicity  of  construction ;  absence  of  all  small  clearances;  absence  of 

thrust  balancing  pistons  with  their  heavy  and  uncertain  leakages. 

(5)  Maintenance  of  efficiency  and  general  durability. 


ALTERNATING-CURRENT  MACHINERY 


171 


(6)  Ability  to  effectively  utilize  the  large  increase  of  available  energy 

incident  to  the  use  of  high  steam  pressure  and  high  vacuum 

(7)  Ability  to  use  high  superheat  without  mechanical  difficulties. 

Fig.  179  is  a  general  view  of  a  Westinghouse-Parsons  three- 
phase  four-pole  turbo-alternator  rated  at  1,000  kilowatts,  60  cycles, 
4,400  volts,  and  1,800  revolutions  per  minute,  as  installed  for  the 
Public  Service  Corporation  of  Columbus,  Ohio.  The  steam  turbine 
is  shown  at  the  right  and  is  directly  connected  through  a  horizontal 
shaft  to  the  alternator  at  the  left.  This  alternator  is  of  the  re- 
volving field  type  as  is  the  universal  practice  with  turbine-driven 
machines. 


Fig.   179.     General  View  of  Westinghouse-Parsona  Three-Phase  Four-Pole  Turbo-Alternator 

Stator.  Except  in  the  largest  sizes,  the  stator  frames  are  made 
in  one  piece.  The  large  machines  are  divided  horizontally,  the  two 
halves  being  bolted  together,  and  the  stator  laminations  are  assem- 
bled as  if  the  frame  were  solid.  The  frame  consists  of  a  heavy  cast- 
ing with  internal  strengthening  ribs  and  is  bored  out  to  receive  the 
laminated  iron  core.  The  internal  ribs  are  provided  with  dovetail 
slots  into  which  the  laminations  are  fitted.  The  frame  is  designed  to 
provide  a  rigid  mechanical  support  for  the  stator  core.  The  cast- 
iron  frame  does  not  form  a  part  of  the  magnetic  circuit.  The  two- 


172 


ALTERNATING-CURRENT  MACHINERY 


part  frames  have  faced  joints  and  the  two  parts  are  secured  and  keyed 
together  so  that  they  form  practically  a  single  piece.  The  armature 
or  stator  is  built  up  of  punchings  of  soft  sheet  steel  of  a  high  magnetic 
quality.  Ventilating  plates  are  provided  at  suitable  intervals,  form- 
ing air  ducts  in  the  core.  The  core  is  slotted  to  receive  the  armature 
winding,  the  shape  of  the  slot  depending  upon  the  capacity  of  the 
machine  and  the  character  of  the  winding.  Either  open  or  partly 
closed  slots  are  used,  the  edges  of  the  former  being  grooved  at  the 
top  to  receive  the  retaining  wedges  holding  the  coils  in  place.  At  the 

ends,  the  teeth  are  supported 
by  finger  plates  and  heavy 
retaining  plates  which  are 
pressed  into  place  and  keyed. 
Closed  end  bells  are  provided 
which  cover  tKe  ends  of  the 
armature  coils  and  the  mov- 
ing parts  of  the  machine  be- 
tween the  frame  and  ven- 
tilating system.  The  end 
bells  close  each  end  of  the 
generator  and  form  a  duct 
through  which  cool  air  is 
drawn  into  the  machine  and 
forced  out  through  ventilat- 
ing ducts  in  the  stator  into 
the  interior  of  the  frame, 
from  which  the  air  passes 
down  through  the  bed  plate 
and  escapes.  In  the  large  generators  the  air  also  escapes  through 
openings  in  the  top  of  the  frame.  The  rotor  of  the  machine  is  pro- 
vided with  a  fan  at  each  end  to  draw  the  air  into  the  machine.  Form- 
wound  armature  coils  are  used  and  the  winding  is  of  wire,  strap,  or 
bar  copper,  depending  upon  the  capacity  and  voltage  of  the  machines. 
The  coils  are  insulated  before  they  are  assembled  on  the  machine. 
The  slots  are  provided  with  a  lining  of  fibrous  material  and  the 
coils  are  wedged  into  the  slots  by  wedges  fitted  in  grooves  in  the 
sides  of  the  slots  or  below  the  overhanging  tips  of  the  teeth.  The 
armature  coils  are  firmly  braced  at  the  ends  of  the  frame  in  such  a 


Fig.   180.     Method  of  Bracing  Armature  Coils 


ALTERNATING-CURRENT  MACHINERY  173 

manner    as   to    insure   them    against    displacement,    as   shown   in 
Fig.  180. 

Rotor.    The  revolving  field  or  rotors  are  constructed  with  two, 
four,  or  six  poles,  according  to  the  frequency  and  capacity  of  the 


Fig.  181.     Four-Pole  Rotor  for  10,000-Kw.  Machine 

machine.  The  fields  are  of  small  diameter  and  are  designed  with 
special  care  to  avoid  windage  losses  and  to  facilitate  ventilation. 
The  poles  of  the  two-pole  and  four-pole  rotors  are  machined  from 
the  disk  or  disks  forming  the  central  body,  and  the  slots  to  receive 
the  field  coils  and  the  grooves  for  the  binding  wedges  are  milled.  This 
construction  is  illustrated  in  Fig.  181,  which  is  a  view  of  a  four-pole 
rotor  for  a  10,000-kilowatt  machine  designed  for  a  speed  of  750 
revolutions  per  minute.  The  six-pole  rotors  are  built  up  by  bolting 
poles  to  a  central  body.  The  rotors  are  carefully  balanced  after 


Fig.   182.     Two-Pole    Rotor    Shdft 


they  are  wound.  In  some  designs  the  rotor  is  pressed  and  keyed 
on  to  the  shaft,  and  in  others  the  shaft  is  formed  of  steel,  cast  or 
forged  integral  with  the  rotor  core.  For  two- pole  machines  the 


174 


ALTERNATING-CURRENT  MACHINERY 


rotor  is  generally  made  from  a  solid  cylinder  and  the  shaft  is  made 
in  two  portions  and  secured  to  each  end  of  the  rotor  by  heavy  bronze 
flanges  and  suitable  bolts,  as  shown  in  Fig.  182. 


Fig.   183.     Curtis  Vertical-Shaft  Turbo-Alternator  of  5,000-Kw.  Capacity 

The  field  consists  of  copper  strap  embedded  in  slots  cut  in  the 
poles,  as  shown  in  Fig.  181.  The  coils  are  wound  directly  in  place 
under  a  heavy  tension.  A  groove  is  cut  in  both  sides  of  the  slots  and 
brass  wedges  are  driven  in  to  hold  the  coils  in  place.  The  coils  are 
heavily  insulated  with  material  of  high  dielectric  and  mechanical 
strength,  applied  in  several  layers.  The  winding  and  insulation  are 
tightly  wedged  into  place. 


ALTERNATING-CURRENT  MACHINERY  175 

Curtis  Turbo- Alternator.  A  general  view  of  a  Curtis  steam  tur- 
bine alternator,  built  by  the  General  Electric  Company,  is  shown 
in  Fig.  183.  This  unit  is  rated  at  5,000  kilowatts,  13,000  volts, 
25  cycles,  and  750  revolutions  per  minute.  As  is  usual  with  Curtis 
turbo-alternators  of  large  size,  this  is  of  the  vertical  type.  The 
generator  is  located  at  the  top  of  the  unit  and  the  turbine  wheels 


Fig.   184.     Two  Views  of  Stationary  Armature  of  Curtis  Alternator 

are  below.    The  stationary  armature  with  the  clamping  devices  for 
rigidly  supporting  the  coils  is  shown  in  Fig.  184. 

Fig.  185  is  a  view  of  a  smooth  core  rotor  for  a  9,000-kilowatt 
vertical  type  of  Curtis  turbo-alternator.  A  feature  of  interest  is 
the  method  employed  for  the  attachment  of  the  shields  for  the  end 
windings.  The  outer  retaining  cylinders  are  secured  to  the  inner 
shell  by  a  large  number  of  short  bolts  which  remove  a  part  of  the 
stress  of  the  end  windings  from  the  outer  shell.  The  rotor  is  so 


176          ALTERNATING-CURRENT  MACHINERY 

designed  that  it  acts  as  a  powerful  fan,  forcing  air  throughout  the 
parts  of  the  generator  requiring  ventilation. 

The  advantages  claimed  for  the  vertical  shaft  type  of  Curtis 
turbine  for  large  machines  are: 

(1)  The  relative  positions  of  revolving  and  stationary  parts  are  definitely 
fixed  by  the  step-bearing. 

(2)  The  stationary  part  is  symmetrical,  and  free  from  distortion  by  heat. 

(3)  The  shaft  bearings  are  relieved  from  all  strain,  and  friction  is  prac- 
tically eliminated. 

(4)  The  shaft  is  free  from  deflection,  and  can  be  made  of  any  size  without 
reference  to  bearings. 


Fig.  185.     Smooth  Core  Rotor  for  9,000-Kw.   Curtis  Turbo-.Alternator 

(5)  The   turbine   structure   affords   support    and   foundation   for   the 
generator. 

(6)  The  cost  of  foundations  is  small,  and  their  support  is  naturally 
simple. 

(7)  Much  floor  space  is  saved. 

(8)  All  parts  of  the  machine  are  accessible. 

For  high  speed  machines  and  turbines  connected  to  direct- 
current  generators,  the  horizontal  shaft  has  some  advantages.  The 
fact  that  many  electric  generators  of  the  horizontal  type  have  already 
been  developed  for  such  conditions  is  frequently  responsible  for 


ALTERNATING-CURRENT  MACHINERY  177 

the  selection  of  the  horizontal  type  of  unit.  In  the  smaller  sizes  this 
type  is  standard.  The  characteristics  of  the  Curtis  turbine  specially 
adapt  it  for  driving  horizontal  shaft  generators.  The  shaft  is  very 
short,  of  small  diameter,  and  has  a  comparatively  low  surface  speed 
in  the  bearings,  resulting  with  the  light  weight  of  the  revolving  parts 
in  low  bearing  friction  and  small  tendency  to  wear. 


ALTERNATING-CURRENT 
MACHINERY 


PART  III 

ECONOMY  FACTORS  IN  ALTERNATORS 
CONDITIONS  AFFECTING  COST 

Speed.  The  most  important  factor  in  determining  the  cost 
of  an  alternator  for  a  given  rated  output,  is  its  speed,  inasmuch  as 
a  low-speed  machine  must  be  much  larger  than  a  high-speed  machine 
of  tne  same  rated  output.  Belt-driven  machines  are  always  run  at  the 
highest  speed  compatible  with  safety  to  the  alternator  itself;  while, 
on  the  other  hand,  the  speed  of  a  direct-connected  alternator,  being 
determined  by  the  speed  of  the  engine  or  water  wheel,  is  usually  less 
than  is  necessary  for  safe  running.  Therefore,  a  belt-driven  machine 
is  usually  cheaper  than  a  direct-connected  machine  of  the  same  rated 
output.  Very  large  machines  must  be  direct-connected,  inasmuch 
as  belt  driving  is  out  of  the  question  for  large  machines  on  account 
of  the  excessive  cost  of  very  large  belts,  the  great  amount  of  floor 
space  required,  the  power  lost  in  the  belt,  the  expense  of  attend- 
ance and  maintenance,  and  the  noise.  Direct-connected  alternators, 
especially  machines  of  large  rated  output,  except  turbo-alternators, 
are  usually  designed  with  the  rotating  member  (armature  or  field) 
of  large  diameter,  in  order  that  the  permissible  speed  of  the  alter- 
nator may  be  approximately  the  same  as  the  proper  speed  of  the 
driving  engine  or  water  wheel.  The  steam  turbine  being  inherently 
a  high-speed  machine  requires  an  alternator  built  for  high  speed, 
which  means  a  rotor  of  small  diameter. 

Voltage.  A  second  factor  affecting  cost  is  the  voltage  that  is  to 
be  developed  in  the  armature.  A  machine  for  high  voltage  must 
have  a  large  number  of  armature  conductors,  and  these  conductors 
must  be  highly  insulated,  that  is,  the  insulation  must  occupy  a 

Copyright,  1912,  by  American  School  of  Correspondence. 


180  ALTERNATING-CURRENT  MACHINERY 

relatively  large  portion  of  the  winding  space  in  the  slots.  This 
requires  a  large  machine  for  a  given  power  output,  on  account  of 
the  space  wasted,  as  it  were,  in  insulation.  To  offset  this  disad- 
vantage, a  high-voltage  alternator  does  not  require  the  use  of  step-up 
transformers,  the  voltage  generated  in  the  alternator  being  suited 
for  the  transmission  of  power  to  moderate  distances,  that  is,  the 
extra  cost  of  the  alternator  may  be  more  than  offset  by  the  saving 
in  the  cost  of  the  step-up  transformers. 

Regulation.  A  third  factor  affecting  cost  is  found  in  the  re- 
quirements of  close  regulation  and  high  efficiency.  Thus  an  alter- 
nator of  given  power  output  may  be  made  smaller  in  size  and,  there- 
fore, cheaper,  if  high  efficiency  and  low  regulation  are  not  demanded. 
High  efficiency  and  low  regulation  mean  a  liberal  use  of  iron  and 
copper  in  order  to  secure  a  minimum  loss  of  power  and  electro- 
motive force  in  a  machine.  Furthermore,  the  efficiency  of  a  given 
size  of  alternator  for  given  output  may  be  increased  at  the  expense 
of  regulation,  or  vice  versa.  The  increasing  adoption  of  a  satisfactory 
automatic  voltage  regulator  for  alternators  makes  it  unnecessary 
as  well  as  expensive  to  specify  a  low  inherent  regulation,  as  explained 
on  page  117. 

Frequency.  A  fourth  factor  affecting  cost  is  the  frequency 
required.  With  given  speed  an  increase  of  frequency  means  an  in- 
crease in  the  number  of  field  poles  and  in  the  number  of  armature 
coils  and,  therefore,  an  increase  in  the  cost  of  construction.  This 
element  of  cost  is  most  prominent  in  very  slow-speed  direct  connected 
alternators.  For  example,  a  60-cycle  alternator,  direct-connected 
to  an  engine  running  at  300  r.  p.  m.,  must  have  24  poles  to  give  the 
required  frequency.  To  reduce  the  frequency  to  25  cycles  would 
require  only  10  poles,  with  a  corresponding  reduction  in  the  cost 
of  the  field-magnet  copper  and  of  labor.  On  the  other  hand,  low- 
ering the  frequency  of  an  alternator,  while  keeping  the  speed  con- 
stant, would  require  an  increase  in  the  useful  flux  per  pole  and  a 
corresponding  increase  in  the  cross-sectional  areas  of  the  field  yoke, 
the  field  poles,  and  the  armature  core.  This  would  mean  an  increase 
in  the  amount  of  iron  to  be  used  in  the  machine.  The  frequencies 
in  most  general  use  today  are  60  cycles  and  25  cycles.  60-cycle  ap- 
paratus is  generally  lower  in  price  and  should  always  be  chosen  for 
general  lighting  and  power  service. 


ALTERNATING-CURRENT  MACHINERY  181 

POWER  LOSSES 

The  power  losses  in  an  alternator  consist  of  the  following  parts: 

(a)  Loss  due  to  brush,  journal,  and  air  friction.    Air  friction  is  usually 
called  windage. 

(b)  Power  consumed  in  heating  the  field  windings  by  the  exciting 
current. 

(c)  Power  loss  in  heating  the  armature  windings  by  the  armature  cur- 
rent or  currents. 

(d)  Hysteresis  loss  in  all  iron  that  is  subject  to  variations  of  magnetiza- 
tion and  eddy  currents  in  all  metal  parts  subject  to  such  variations. 

Friction  and  windage  loss  can  be  determined  only  by  experi- 
ments upon  the  finished  machine. 

The  power  consumed  in  the  field  windings  is  I2R,  where  I  is 
the  field  current  and  R  is  the  resistance  of  the  field  circuit.  The 
field  rheostat  is  properly  a  part  of  the  machine,  and  the  losses  occur- 
ring in  it  are,  therefore,  a  part  of  the  machine  losses.  This  same 
formula  may  be  used  to  calculate  power  consumed  in  each  field  wind- 
ing of  a  composite-field  alternator. 

The  power  lost  in  heating  the  armature  windings  is  PR  X  the 
number  of  phases,  where  7  is  the  current  in  each  phase  and  R  is  the 
resistance  of  each  phase. 

The  power  lost  by  eddy  currents  and  hysteresis  may  be  ap- 
proximately calculated  by  the  method  employed  for  the  corre- 
sponding calculation  in  the  case  of  a  transformer. 

EFFICIENCY 

The  efficiency  of  an  alternator  is  the  ratio  output  of  power  -*-  input 
of  power.  Since  the  mechanical  input  of  power  is  equal  to  the  output 
of  power  plus  all  the  losses  of  power,  we  have  also 

~  .  output 

efficiency  =  - 


output  +  losses 

At  zero  load  (zero  output),  the  efficiency  of  an  alternator  is,  there- 
fore, zero;  the  efficiency  increases  with  increasing  load,  reaches  a 
maximum,  and  falls  off  for  large  loads.  An  alternator  may  be  de- 
signed to  give  its  maximum  efficiency  at  any  prescribed  fraction 
of  full  load;  it  is  generally  desirable,  however,  to  design  the  machine 
to  give  its  maximum  efficiency  at  approximately  full  load. 


182 


ALTERNATING-CURRENT  MACHINERY 


The  efficiency  of  a  large  alternator  at  full  load  is  usually  greater 
than  the  efficiency  of  a  small  alternator.  For  example,  the  large 
alternators  in  the  great  power  stations  at  Niagara  Falls  have  effi- 
ciencies of  over  98  per  cent.  A  well-designed  50-kilowatt  alternator 
has  an  efficiency  of  about  90  per  cent. 

Practical  and  Ultimate  Limits  of  Output.  The  dotted  curve, 
Fig.  186,  is  the  characteristic  curve  of  a  given  alternator.  This 
curve  shows  the  relation  between  the  current  output  (plotted  as 
abscissas)  and  the  electromotive  force  between  the  collecting  rings 
(plotted  as  ordinates,  using  scale  to  the  left),  the  field  excitation 
being  kept  constant.  The  "ordinates  of  the  full-line  curve  (scale 
shown  to  the  right  in  the  figure)  represent  the  power  outputs  (in 
kilowatts)  corresponding  to  the  different  current  outputs  assuming 

a  non-inductive  receiving 
circuit.  The  maximum  out- 
put of  the  alternator,  in  this 
case,  is  68  kilo  watts  when  the 
current  output  is  38  amperes 
and  the  corresponding  electro- 
motive force  is  1,790  volts. 
In  practice  the  allowable 
power  output  of  an  alternator 
is  limited  to  a  smaller  value 
than  this  maximum  output  by 
one  or  the  other  of  the  follow- 
ing considerations: 


2400 
220  C 
2000 
1800 
1600 

I  £00 
L_ 

^1200 
U 

•J  000 

-^ 

-L 

70 

60 
50* 
40 
30 
20 
10 

i 

^ 

X. 

. 

*^ 

. 

^ 

\    - 

\ 

/ 

^^" 

\\ 

I/ 

r 

\ 

^J 

/ 

[ 

\ 

/ 

/ 

/ 

2 

Fig.   186. 


15    20    25     30     35    40     45     50 
Current 


Characteristic    Curves    of    an 
Alternator 


(a)  Electric  lighting  and  power 
service  usually  demands  an  approx- 
imately constant  electromotive 
force;  and  it  is  not  permissible  to  take  from  an  alternator  a  current  so  large 
as  greatly  to  reduce  its  electromotive  force.  This  difficulty  may  be  largely 
overcome  by  providing  for  an  increase  of  field  excitation  of  the  alternator 
with  increase  of  load,  as  is  done  in  the  alternator  with  a  composite  field 
winding,  or  by  means  of  the  Tirrill  voltage  regulator. 

(b)  The  current  delivered  by  an  alternator  generates  heat  in  the  arma- 
ture of  the  alternator;  and  the  temperature  of  the  armature  rises  until  it  radiates 
heat  as  fast  as  heat  is  generated  in  it  by  the  current.  Excessive  heating  of 
the  armature  endangers  the  insulation  of  the  windings  by  charring,  and  it  is 
not  permissible  to  take  from  an  alternator  a  current  so  large  as  to  heat  its 
armature  more  than  40°  or  50°  C.  above  the  temperature  of  the  air;  this 
heating,  therefore,  fixes  the  allowable  output  and  rating  of  an  alternator. 


ALTERNATING-CURRENT  MACHINERY  183 

Influence  of  Power  Factor  upon  Output.  Alternators  are  rated 
according  to  the  power  they  can  deliver  steadily  to  a  non-inductive 
receiving  circuit  without  overheating.  The  amount  of  power  which 
an  alternator  can  satisfactorily  deliver  to  an  inductive  receiving 
circuit  is  less  than  that  which  it  can  deliver  to  a  non-inductive 
receiving  circuit,  because  of  the  phase  difference  of  electromotive 
force  and  current.  The  cosine  of  the  angle  of  phase  difference  (cos  6) 
is  called  the  "power  factor"  of  the  receiving  circuit,  as  before  pointed 
out.  The  power  factor  of  incandescent  lighting  circuits  is  very 
nearly  unity  if  the  transformers  are  all  operating  at  approximately 
full  load. 

A  transformer  having  its  primary  coil  connected  to  alternating- 
current  mains,  but  furnishing  no  current  from  its  secondary  coil, 
has  a  power  factor  of  from  about  0.3  with  a  frequency  of  25  cycles 
to  about  0.7  with  60  cycles.  The  power  factor  increases  with  the 
secondary  lamp  load,  until,  in  the  neighborhood  of  full  load,  the 
power  factor  is  nearly  unity. 

Induction  motors,  like  transformers,  have  a  maximum  power 
factor  at  full  load,  which  rarely  exceeds  0.9;  this  factor  under  partial 
loads  falls  to  0.7  or  even  less;  and  at  starting  it  is  a  minimum  around 
0.3. 

A  mixed  load  consisting  of  transformers  and  induction  motors 
more  or  less  fully  loaded,  has  an  average  power  factor  of  from  0.8 
to  0.85. 

It  should  be  carefully  noted  that  alternators  are  rated  in  kilo  volt- 
ampere  output,  that  is,  a  100-k.v.a.  alternator  should  deliver 
its  full  energy  output  of  100  kilowatts  at  unity  power  factor;  but  if 
the  power  factor  should  be  only  0.8,  the  true  power  output  would 
be  reduced  to  80  kilowatts,  although  the  armature  current  and  the 
I2R  loss  would  be  about  the  same  as  if  the  machine  were  delivering 
100  kilowatts  at  unity  power  factor.  To  find  the  actual  power  output 
of  an  alternator  the  "apparent  power,"  or  kilo  volt-amperes,  must 
be  multiplied  by  the  power  factor  of  the  load. 

RATING  AND  OVERLOAD  CAPACITIES 

Previous  to  the  definite  recommendations  of  the  American 
Institute  of  Electrical  Engineers  in  the  matter  of  overload  capacities 
as  given  below,  there  were  great  differences  among  different  manu- 


184  ALTERNATING-CURRENT  MACHINERY 

facturers  in  the  rating  of  alternators.  Thus,  one  maker  might  have 
sold  a  certain  alternator  as  a  50-kilowatt  alternator,  although  the 
machine  might  have  been  capable  of  delivering  50  per  cent  overload 
(or  75  kilowatts)  for  several  hours  without  dangerous  rise  of  tem- 
perature; whereas  another  maker  might  have  sold  an  exactly  similar 
machine  as  a  75-kilowatt  alternator. 

As  an  illustration  of  the  difference  of  permissible  rating  of  a 
given  alternator  according  to  conditions  of  service,  the  following 
is  taken  from  the  practice  of  one  of  the  large  American  manufac- 
turing companies. 

A  certain  alternator  has  twenty  poles,  and  runs  at  150  r. p.m.  When 
this  machine  is  rated  at  300  kilowatts,  its  maximum  rise  of  temperature  will 
not  exceed  35 °C.  (by  thermometer)  when  it  is  operated  continuously  with 
non-inductive  full  load;  its  maximum  rise  of  temperature  will  not  exceed  55° C. 
when  it  is  run  for  two  hours  at  50  per  cent  overload  non-inductive.  Its  full- 
load  regulation  will  be  6  per  cent  with  non-inductive  load,  and  18  per  cent  with 
80  per  cent  power  factor. 

When  this  same  machine  is  rated  at  360  kilowatts,  its  maximum  rise 
of  "temperature  will  not  exceed  40  °C.  (by  thermometer)  when  it  is  operated 
continuously  with  non-inductive  full  load;  its  maximum  rise  of  temperature 
will  not  exceed  55  °C.  when  it  is  run  for  two  hours  at  25  per  cent  overload 
non-inductive.  Its  full  load  regulation  will  be  8  per  cent  with  non-inductive 
load,  and  22  per  cent  with  80  per  cent  power  factor. 

In  general  the  rated  power  output  of  any  alternator  may  be 
20  per  cent  higher  with  a  permissible  rise  of  temperature  of  40°C. 
and  a  regulation  of  8  per  cent  on  non-inductive  full  load,  than  with 
a  permissible  rise  of  temperature  of  35°C.  and  a  regulation  of  6  per 
cent  on  non-inductive  full  load. 

American  Institute  Rules.  In  order  to  establish  a  definite  and 
uniform  basis  for  rating  alternators  and  for  guaranteeing  their  per- 
formance in  service  under  normal  full  load  as  well  as  overload,  the 
following  rules  have  been  adopted  by  the  American  Institute  of 
Electrical  Engineers.  The  numbers  on  the  left  are  the  paragraph 
numbers  of  the  revised  (1907)  report  of  the  Institute  Committee  on 
Standardization. 

RATING 

65  RATING  BY  OUTPUT.     All  electrical  apparatus  should  be  rated  by  output 
and  not  by  input.     Generators,  transformers,  etc.,  should  be  rated  by 
electrical  output;  motors  by  mechanical  output. 

66  RATING  IN  KILOWATTS.     Electrical  power  should  be  expressed  in  kilo- 
watts, except  when  otherwise  specified. 


ALTERNATING-CURRENT  MACHINERY  185 

67  APPARENT  POWER,   KILOVOLT-AMPERES.    Apparent  power    in    alter- 
nating-current circuits  should  be  expressed  in  kilovolt-amperes  as  distin- 
guished from  real  power  in  kilowatts.    When  the  power  factor  is  100  per 
cent,  the  apparent  power  in  kilovolt-amperes  is  equal  to  the  kilowatts. 

68  RATED  (FULL-LOAD)  CURRENT  is  that  current  which,  with  the  rated 
terminal  voltage,  gives  the  rated  kilowatts,  or  the  rated  kilovolt-amperes. 
In  machines  in  which  the  rated  voltage  differs  from  the  no-load  voltage, 
the  rated  current  should  refer  to  the  former. 

69  DETERMINATION  OF  RATED  CURRENT.     The  rated  current  may  be  de- 
termined as  follows :    If  P  =  rating  in  watts,  or  apparent  watts  if  the  power 
factor  be  other  than  100  per  cent,  and  E  =  full-load  terminal  voltage, 
the  rated  current  per  terminal  is : 

p 

70  7  =  —  in  a  direct-current  machine  or  single-phase  alternator. 

E 

I          p 

71  /  =    • —  X —  m  a  three-phase  alternator. 

1  P 

72  I  =  —  X  ~^T  in  a  two-phase  alternator. 

2  h 

73  NORMAL  CONDITIONS.     The  rating  of  machines  or  apparatus  should  be 
based  upon  certain  normal  conditions  to  be  assumed  as  standard,  or  to  be 
specified.     These  conditions  include  voltage,  current,  power-factor,  fre- 
quency, wave  shape  and  speed;  or  such  of  them  as  may  apply  in  each  par- 
ticular case.     Performance  tests  should  be  made  under  these  standard 
conditions  unless  otherwise  specified. 

74  (a)     Power  Factor.     Alternating-current  apparatus  should  be  rated  in 
kilowatts,  at  100  per  cent  power  factor;  i.  e.,  with  current  in  phase  with 
terminal  voltage,  unless  a  phase  displacement  is  inherent  in  the  apparatus 
or  is  specified.     If  a  power  factor  other  than  100  per  cent  is  specified, 
the  rating  should  be  expressed  in  kilovolt-amperes  and  power  factor,  at 
rated  load. 

75  (b)     Wave  Shape.     In  determining  the  rating  of  alternating-current 
machines  or  apparatus,  a  sine  wave  shape  of  alternating  current  and 
voltage  is  assumed,  except  where  a  distorted  wave  shape  is  inherent  to 
the  apparatus. 

LIMITING  TEMPERATURE  RISE 

272  GENERAL.     The   temperature   of   electrical   machinery   under  regular 
service  conditions,  should  never  be  allowed  to  remain  at  a  point  at  which 
permanent  deterioration  of  its  insulating  material  takes  place. 

273  LIMITS  RECOMMENDED.     It  is  recommended  that  the  following  maxi- 
mum values  of  temperature  elevation,  referred  to  a  standard  room  tem- 
perature of  25  degrees  centigrade,  at  rated  load  under  normal  conditions 
of  ventilation  or  cooling,  should  not  be  exceeded. 

274  (A)     MACHINES  IN  GENERAL.     In  commutating  machines,  rectifying 
machines,  pulsating-current  generators,   synchronous  machines,  synchro- 
nous commutating  machines  and  unipolar  machines,  the  temperature  rise 
in  the  parts  specified  should  not  exceed  the  following: 


186          ALTERNATING-CURRENT  MACHINERY 

275  Field  and   armature,   50°C. 

276  Commutator  and  brushes,  by  thermometer,  55 °C. 

277  Collector  rings,  65 °C. 

278  Bearings  and  other  parts  of  machine,  by  thermometer,  40 °C. 

279  (B)    ROTARY  INDUCTION  APPARATUS.      The  temperature  rise  should 
not  exceed  the  following: 

280  Electric  circuits,  50 °C.,  by  resistance. 

281  Bearings  and  other  parts  of  the  machine  40°  C.,  by  thermometer. 

282  In  squirrel-cage  or  short-circuited  armatures,  55°  C.,  by  thermometer, 
1   may  be  allowed. 

(C)    STATIONARY  INDUCTION  APPARATUS. 

283  (a)     Transformers     for     Continuous     Service.    The     temperature    rise 
should  not  exceed  50  degrees  centigrade  in  electric  circuits,  by  resistance; 
and  in  other  parts,  by  thermometer. 

284  (b)     Transformers  for  Intermittent  Service.  In  the  case  of  transformers 
intended  for  intermittent  service,  or  not  operating  continuously  at  rated 
load,  but  continuously  in  circuit,  as  in  the  ordinary  case  of  lighting  trans- 
formers, the  temperature  elevation  above  the  surrounding  air-temperature 
should  not  exceed  50°  C.,  by  resistance  in  electric  circuits  and  by  ther- 
mometer in  other  parts,  after  the  period  corresponding  to  the  term  of  rated 
load.    In  this  instance,  the  test  load  should  not  be  applied  until  the  trans- 
former has  been  in  circuit  for  a  sufficient  time  to  attain  the  temperature 
elevation  due  to  core  loss.     With  transformers  for  commercial  lighting, 
the  duration  of  the  rated-load  test  may  be  taken  as  three  hours,  unless 
otherwise  specified. 

285  (c)     Reactors,  induction-  and  magneto-regulators — electric  circuits  by 
resistance  and  other  parts  by  thermometer,  50 °C. 

286  (d)     Large  Apparatus.   Large  generators,  motors,  transformers,  or  other 
apparatus  in  which  reliability  and  reserve  overload  capacity  are  important, 
are  frequently  specified  not  to  rise  in  temperature  more  than  40°  C.  under 
rated  load  and  55°  C.  at  rated  overload.     It  is,  however,  ordinarily  un- 
desirable to  specify  lower  temperature  elevations  than  40  degrees  centi- 
grade at  rated  load,  measured  as  above. 

(E)    LIMITS  RECOMMENDED  IN  SPECIAL  CASES. 

289  (a)      Heat  Resisting  Insulation.     With  apparatus  in  which  the  insula- 
ting materials  have  special  heat-resisting  qualities,  a  higher  temperature 
elevation  is  permissible. 

290  (b)      High  Air  Temperature.      In   apparatus    intended   for   service  in 
places  of  abnormally  high  temperature,  a  lower  temperature   elevation 
should  be  specified. 

291  (c)     Apparatus  Subject  to  Overload.     In  apparatus  which  by  the  nature 
of  its  service  may  be  exposed  to  overload,  or  is  to  be  used  in  very  high 
voltage  circuits,  a  smaller  rise  of  temperature  is  desirable  than  in  apparatus 
not  liable  to  overloads  or  in  low- voltage  apparatus.     In  apparatus  built 
for  conditions  of  limited  space,  as  railway  motors,  a  higher  rise  of  tem- 
perature must  be  allowed. 

292  (d)      Apparatus  for  Intermittent   Service.     In    the    case    of    apparatus 
intended  for  intermittent  service,  except  railway  motors,  the  temperature 


ALTERNATING-CURRENT  MACHINERY  187 

elevation  which  is  attained  at  the  end  of  the  period  corresponding  to  the 
term  of  rated  load,  should  not  exceed  the  values  specified  for  machines 
in  general.  In  such  apparatus  the  temperature  elevation,  including  rail- 
way motors,  should  be  measured  after  operation,  under  as  nearly  as  pos- 
sible the  conditions  of  service  for  which  the  apparatus  is  intended,  and 
the  conditions  of  the  test  should  be  specified. 

OVERLOAD  CAPACITIES 

293  PERFORMANCE  WITH  OVERLOAD.     All  apparatus  should  be  able  to  carry 
the  overload  hereinafter  specified  without  serious  injury  by  heating,  spark- 
ing, mechanical  weakness,  etc.,  and  with  an  additional  temperature  rise 
not  exceeding  15°C.,  above  those  specified  for  rated  loads,  the  overload 
being  applied  after  the  apparatus  has  acquired  the  temperature  corre- 
sponding to  rated  load  continuous  operation.     Rheostats  to  which  no 
temperature  rise  limits  are  attached  are  naturally  exempt  from  this  addi- 
tional|temperature  rise  of  15°C.  under  overload  specified  in  these  rules. 

294  NORMAL  CONDITIONS.     Overload   guarantees    should  refer  to   normal 
conditions  of  operation  regarding  speed,  frequency,  voltage,  etc.,  and  to 
non-inductive  conditions  in  alternating  apparatus,  except  where  a  phase 
displacement  is  inherent  in  the  apparatus. 

295  OVERLOAD  CAPACITIES  RECOMMENDED.     The  following  overload  capaci- 
ties are  recommended. 

296  (a)     Generators.     Direct-current     generators     and    alternating-current 
generators,  25  per  cent  for  two  hours. 

297  (b)     Motors.     Direct-current  motors,  induction  motors  and  synchronous 
motors,  not  including  railway  and  other  motors  intended  for  intermittent 
service,  25  per  cent  for  two  hours,  and  50  per  cent  for  one  minute. 

298  (c)     Converters.     Synchronous  converters,  25  per  cent  for  two  hours, 
50  per  cent  for  one-half  hour. 

299  (d)     Transformers     and     Rectifiers.     Constant-potential     transformers 
and  rectifiers,  25  per  cent  for  two  hours;  except  in  transformers  connected 
to  apparatus  for  which  a  different   overload  is    guaranteed,    in    which 
case  the  same  guarantees  shall  apply  for  the  transformers  as  for  the 
apparatus  connected  thereto. 

300  (e)     Exciters.     Exciters  of  alternators  and  other  synchronous  machines, 
10  per  cent  more  overload  than  is  required  for  the  excitation  of  the  syn- 
chronous machine  at  its  guaranteed  overload,  and  for  the  same  period  of 
time.    All  exciters  of  alternating-current,  single-phase,  or  polyphase  gen- 
erators should  be  able  to  give  at  its  rated  speed,  sufficient  voltage  and 
current  to  excite  the  alternator,  at  the  rated  speed,  to  the  full-load  ter- 
minal voltage,  at  the  rated  output  in  kilovolt-amperes  and  with  50  per 
cent   power  factor. 

301  (f)     A    Continuous-Service   Rheostat,   such   as   an   armature-  or  field- 
regulating  rheostat,  should  be  capable  of  carrying  without  injury  for  two 
hours,  a  current  25  per  cent  greater"  than  that  at  which  it  is  rated.     It 
should  also  be  capable  of  carrying  for  one  minute  a  current  50  per  cent 
greater  than  its  rated  load  current,  without  injury.     This  excess  of  ca- 
pacity is  intended  for  testing  purposes  only,  and  this  margin  of  capacity 
should  not  be  relied  upon  in  the  selection  of  the  rheostat. 


188  ALTERNATING-CURRENT  MACHINERY 

ALTERNATOR  TESTING 

Alternating=Current  Testing  in  General.  In  the  commercial 
testing  of  alternating-current  apparatus,  as  in  that  of  direct-current 
apparatus,  the  object  of  the  tests  is  to  determine  the  performance 
of  the  apparatus  under  normal  working  conditions.  Care  must  be 
taken,  therefore,  to  carry  out  each  test  under  the  normal  work- 
ing conditions  with  respect  to  speed,  voltage,  frequency,  etc.  Errors 
of  observation  may  be  greatly  reduced  by  taking  a  series  of  observa- 
tions instead  of  a  single  observation  or  a  single  set  of  observations. 
The  observations  of  this  series  should  be  plotted  point  for  point, 
and  a  smooth  curve  drawn  through  the  points  in  such  a  way  that 
the  points  will  be  equally  distributed  on  both  sides  of  the 
curve. 

Different  classes  of  apparatus  require  different  tests;  more- 
over, the  same  test  may  be  performed  differently  on  different  kinds 
of  machines.  For  the  sake  of  convenience  the  tests  necessary  for 
each  class  of  alternating-current  apparatus  will  be  considered  under 
its  own  heading. 

In  general  every  piece  of  electrical  apparatus  must  satisfy  two 
vital  requirements,  namely,  (a)  it  must  have  insulation  of  sufficient 
strength  to  stand  safely  the  voltage  at  which  it  is  intended  to  be 
operated;  and  (b)  it  must  not  overheat  under  normal  working  con- 
ditions. 

Faulty  insulation  must  be  carefully  guarded  against,  since,  by 
the  breaking  down  of  the  insulation,  the  apparatus  must  be  put  out 
of  service.  For  example,  in  the  case  of  transformers  for  lighting 
service,  a  breakdown  of  the  insulation  between  the  primary  and  the 
secondary  coils  endangers  the  life  of  persons  coming  in  contact  with 
fixtures  on  the  secondary  circuit.  Overheating  a  machine  causes  a 
gradual  deterioration  of  the  insulation,  which  may  finally  result  in 
a  complete  breakdown  of  the  apparatus.  Overheating  must,  there- 
fore, be  avoided. 

Insulation  Testing.  There  are  three  distinct  kinds  of  insula- 
tion tests:  (a)  the  determination  of  the  electrical  resistance  of  the 
insulation  in  ohms;  (b)  the  subjecting  of  the  insulation  of  an  apparatus 
to  a  prescribed  voltage  in  excess  of  the  rated  voltage  of  the  apparatus. 
This  test  is  intended  to  insure  that  the  apparatus  will  operate  safely 
at  its  rated  voltage.  It  is  frequently  called  the  "test  of  dielectric 


ALTERNATING-CURRENT  MACHINERY  189 

strength;"  and  (c)  the  subjecting  of  the  insulation  to  a  voltage  which  is 
.increased  until  the  insulation  is  punctured  or  breaks  down.  This  is 
called  the  "break-down  test." 

Insulation  Resistance.  The  insulation  resistance  test,  or  in  other 
words,  the  determination,  in  ohms,  of  the  resistance  of  an  insulating 
coating,  cover,  material,  or  support  is  usually  made  by  measuring 
with  a  sensitive  galvanometer  the  very  small  current  that  is  forced 
through  the  insulation  by  a  known  direct  or  steady  electromotive 
force.  The  value  of  the  resistance  is  equal  to  the  impressed  electro- 
motive force  divided  by  the  current. 

The  resistance  in  ohms  of  the  insulation  is  of  only  secondary 
importance  as  compared  with  the  dielectric  strength  or  the  resist- 
ance to  rupture  by  high  voltage.  Insulation  resistance  tests  should, 
if  possible,  be  made  using  the  electromotive  force  for  which  the 
apparatus  is  designed. 

The  insulation  resistance  of  the  complete  apparatus  should  be 
such  that  the  rated  voltage  of  the  apparatus  will  not  send  more 
than  -nnrTTnnr  of  the  full  load  current,  at  the  rated  terminal  volt- 
age, through  the  insulation.  Where  the  value  found  in  this  way  is 
more  than  1  megohm,  it  is  usually  sufficient. 

Dielectric  Strength.  The  test  of  dielectric  strength  may  be  made 
as  follows:  The  terminals  of  the  secondary  coil  of  a  step-up  "high 
potential"  transformer  are  connected  to  the  terminals  of  a  spark 
gauge  and  to  the  terminals  of  the  insulation  to  be  tested,  as  shown 
in  Fig.  187.  The  figure  shows  that  the  apparatus  under  test  and 
the  spark  gap  are  connected  in  parallel  between  the  terminals  of 
the  secondary  (high  voltage)  coil  of  the  testing  transformer.  The 
spark  gauge,  page  95,  is  set  at  a  sparking  distance  corresponding 
to  the  voltage  to  be  used  in  the  test;  and  the  voltage  is  increased 
gradually  by  adjusting  the  water  rheostat  in  series  with  the  pri- 
mary (low-voltage)  coil  of  the  transformer,  and  is  then  to  be  con- 
tinuously applied  for  a  prescribed  period,  usually  one  minute, 
until  either  the  insulation  is  punctured  or  the  desired  voltage  is 
reached,  as  will  be  indicated  by  the  sparking  across  the  gap  between 
the  needle  points. 

Fig.  187  shows  the  electrical  connections  for  carrying  out  a 
test  of  dielectric  strength  on  a  certain  commercial  transformer, 
marked  in  the  figure  "apparatus  under  test."  It  will  be  seen  that 


190 


ALTERNATING-CURRENT  MACHINERY 


one  terminal  of  the  high-voltage  coil  of  the  testing  transformer  is 
connected  to  one  terminal  of  the  primary  coil  (at  the  left)  of  the 
transformer  under  test,  and  the  other  terminal  of  the  high-voltage 
coil  of  the  testing  transformer  is  connected  to  one  of  the  four  ter- 
minals of  the  secondary  coils  of  the  transformer  under  test.  The 
high  voltage  is,|therefore,  applied  to  the  insulation  between  the  pri- 
mary and  secondary  coils  of  the  transformer  under  test. 


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Fig.   187.     Diagram  of  Connections  for  Dielectric  Strength  Test  with  High 
Potential  Transformer 


The  use  of  a  water  rheostat  in  series  with  the  primary  (low 
voltage)  coil  of  the  testing  transformer,  as  shown  in  Fig.  187,  for 
varying  the  impressed  voltage,  is  open  to  some  objection.  Such  a 
resistance  is  liable  to  seriously  affect  the  wave  form  of  the  electro- 
motive force,  thereby  causing  its  maximum  value  to  bear  a  different 
and  unknown  ratio  to  its  effective  value.  The  most  approved  way  ' 
of  securing  voltage  control  is  by  adjusting  the  field  excitation  of  the 
alternator  supplying  the  testing  transformer  with  current.  Another 
method,  though  not  as  satisfactory,  is  by  using  a  transformer  with 
a  variable  ratio  of  primary  to  secondary  turns. 


ALTERNATING-CURRENT  MACHINERY 


191 


Tests  of  dielectric  strength  are  made  with  voltages  ranging 
from  1  \  to  10  times  the  rated  terminal  voltage  of  a  piece  of  apparatus, 
according  to  the  rated  voltage  and  output  of  the  apparatus.  For 
example,  a  transformer  of  any  output  whose  rated  terminal  vol- 
tage is  20,000  would,  according  to  the  recommendations  of  the 
American  Institute  of  Electrical  Engineers,  be  tested  with  40,000 
volts,  whereas  a  1,000-volt  transformer  would  be  tested  with  3,500 
volts.  An  induction  motor  under  10  h.  p.  rated  at  110  volts  would 
be  tested  with  1,000  volts. 

Break-Down.    The  break-down  test  is  freauently  applied  in  the 


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15 
5 
55 
35 

15 
9 
75 
85 

95 
0 

-^ 

/ 

y/ 

f 

/ 

^ 

/ 

f 

/ 

JL 
+t. 

J>< 
z/ 

8. 
9.1 
/O 
/J. 

/3 

/J 

f 

/ 

A 

/ 

f 

/ 

f 

/ 

j 

/ 

^ 

>^ 

s 

/ 

^ 

X 

^X 

^ 

/O      ^0    JO     4O     SO      6O     70     80     90     /OO    //O 
MLO~VOLT3-SQUA/?C  /TOOT 


/3O    /4O   /50 


Fig.   188.     Curve  Showing  Relation  between  Sparking  Distance  and  Voltage 

testing  of  small  samples  of  insulating  material.  For  example,  a  sheet 
of  fuller  board,  mica-canvas,  oiled  linen  or  cloth,  would  be  clamped 
between  sheets  of  metal  connected  to  the  terminals  of  the  high- 
voltage  coil  of  the  testing  transformer,  and  the  voltage  would  be 
increased  until  the  insulation  was  punctured,  the  voltage  causing 
puncture  being  recorded.  A  basis  is  thus  obtained  for  the  acceptance 
or  rejection  by  the  purchaser  of  a  lot  of  insulating  material. 

Table  VI  gives,  in  inches  and  in  centimeters,  the  sparking 
distances  in  air  between  opposed  sharp  needle-points,  correspond- 
ing to  various  effective  sinusoidal  voltages.  The  voltages  are  ex- 
pressed in  kilo  volts  (1  kilo  volt  =  1,000  volts).  Fig.  188  is  a  curve 


192 


ALTERNATING-CURRENT  MACHINERY 


TABLE  VI 
Sparking  Distances  for  Various  Voltages 


Kilovolts 
Square  Root  of 
Mean  Square 

Distance 

Kilovolts 
Square  Root  of 
Mean  Square 

Distance 

Inches            Cms. 

Inches            Cms. 

5 

0.225      0.57 

140 

13.95      35.4 

10 

0.47         1.19 

150 

15.0        38.1 

15 

0.725       1.84 

160 

16.05      40.7 

20 

1.0          2.54 

170 

17.10      43.4 

25 

1.3           3.3 

180 

18.15       46.1 

30 

1.625       4.1 

190 

19.20       48.8 

35 

2.0           5.1 

200 

20.25       51.4 

40 

2.45         6.2 

210 

21.30       54.1 

45 

2.95         7.5 

220 

22.35       56.8 

50 

3.55         9.0 

230 

23.40       59.4 

60 

4.65       11.8 

240 

24.45       62.1 

70 

5.85       14.9 

250 

25.50       64.7 

80 

7.1         18.0 

260 

26.50       67.3 

90 

8.35       21.2 

270 

27.50       69.8 

100 

9.6         24.4 

280 

28.50       72.4 

110 

10.75      27.3 

290 

29.50       74.9 

120 

11.85      30.1 

300 

30.50       77.4 

130 

12.90      32.8 

plotted  from  the  data  given  in  the  table  and  shows  graphically  the 
relation  between  sparking  distance  and  voltage.  The  voltage  corre- 
sponding to  a  given  sparking  distance  varies  greatly  with  the  sharp- 
ness of  the  needle-points,  and  with  the  shape  of  the  electromotive 
force  wave. 

Sometimes  the  value  of  the  voltage  applied  to  the  apparatus 
is  not  measured  by  the  spark  gauge,  but  is  inferred  from  the  read- 
ing of  a  low-reading  voltmeter  connected  between  the  points  A 
and  B  in  Fig.  187.  Thus,  if  there  are  100  times  as  many  turns 
of  wire  in  the  secondary  (high-voltage)  coil  of  the  testing  trans- 
former as  in  the  primary  coil,  then  the  readings  of  the  voltmeter 
connected  as  specified  must  be  multiplied  by  100.  The  best  prac- 
tice is  to  connect  an  electrostatic  voltmeter  in  parallel  with  the  spark 
gap  and  to  check  its  readings  against  the  sparking  distances  in  accord- 
ance with  Table  VI.  It  should  be  noted,  however,  that  the  volt- 
meter gives  effective  values  while  the  spark  gap  gives  maximum 
values  of  the  voltage. 


ALTERNATING-CURRENT  MACHINERY 


193 


Characteristic  Curves.  Saturation  Curve.  The  saturation  curve 
of  a  generator  shows  the  relation  between  the  volts  generated  in 
the  armature  and  the  amperes  of  field  current  (or  ampere-turns  of 
the  field)  for  a  constant  armature  current.  The  armature  current 
may  be  zero,  in  which  case  the  curve  is  called  the  no-load  saturation 
curve,  or  sometimes  the  open-circuit  characteristic  curve.  Observa- 
tions for  a  saturation  curve  may  be  taken  with  full-load  current 
in  the  armature;  but  this  is  rarely  done,  except  in  alternators  of 
comparatively  small  output.  If  a  full-load  saturation  curve  is 
desired,  it  can  be  approximately  calculated  from  the  no-load  satura- 
tion curve  and  the  curve  for  synchronous  impedance,  as  will  be 
explained  later. 


Fig.   189.     Diagram  of  Connections  for  Saturation  Curve  Test  of 
Alternator 

The  diagram  of  connections  for  determining  tne  no-load  satura- 
tion curve  is  shown  in  Fig.  189.  The  alternator  is  represented  as 
a  three-phase  machine  of  the  revolving-field  type  (armature  is 
stationary).  The  field  winding  is  connected  through  slip-rings 
d  and  e,  and  brushes  /  and  g,  to  an  ammeter  A,  and  through  an 
adjustable  resistance  r,  to  the  exciter  or  other  direct-current  source. 
A  voltmeter  V\  is  connected  across  the  field-winding  terminals 
to  measure  the  voltage  applied  to  the  field  winding.  The  volt- 
meter F2,  for  measuring  the  voltage  generated  by  the  machine, 
is  connected  directly  to  two  of  the  armature  terminals,  a  and  c,  in 
the  case  of  a  low- voltage  machine.  If  the  voltage  generated  is 
greater  than  the  capacity  of  the  voltmeter,  a  multiplying  coil  or  a 


194 


ALTERNATING-CURRENT  MACHINERY 


vo/fage 
oadf/eh 


step-down  potential  transformer  may  be  used  to  reduce  the  electro- 
motive force  to  be  measured.  For  very  high  voltages  a  potential 
transformer  must  be  used. 

A  series  of  observations  of  the  electromotive  force  between 
the  terminals  of  one  of  the  phases,  such  as  a  and  c,  is  made  for 

different  values  of  the 
field  current.  Eight  or 
nine  points  along  the 
curve  are  usually  suf- 
ficient, the  series  ex- 
tending from  zero  elec- 
tromotive force  to 
about  fifty  per  cent 
above  normal  rated 
voltage.  The  points 
should  be  taken  more 
closely  together  in  the 
vicinity  of  normal  volt- 
age than  at  other  por- 
tions of  the  curve.  Care 
must  be  taken  that  the 
generator  is  run  at  its 
rated  speed,  and  this 
speed  must  be  kept  con- 
stant. Deviations  from 
constant  speed  may  be 

Fig.  190.     Saturation  Curve  for  2000-Kw.  Alternator       most  easily   detected   by 

the  use  of  a  tachometer. 

If  the  machine  is  two-phase  or  three-phase,  the  voltmeter  may 
be  connected  to  any  one  phase  throughout  a  complete  series  of 
observations.  The  voltage  of  all  the  phases  should  be  observed 
for  normal  full-load  excitation  by  connecting  the  voltmeter  to  each 
phase  successively,  keeping  the  field  current  constant  at  normal 
voltage.  This  is  done  in  order  to  see  how  closely  the  voltage  of 
the  different  phases  agree. 

The  observations  required  for  the  determination  of  the  satura- 
tion curve  are  volts  at  armature  terminals,  Vi',  amperes  in  field,  A; 
volts  at  field  terminals,  V\\  and  speed. 


•urar/on  Cu 


for 
W  A/rerr 


afor 


ALTERNATING-CURRENT  MACHINERY 


195 


Fig.  190  shows  a  saturation  curve  taken,  from  a  2,000-kilowatt 
three-phase  alternator  of  the  revolving-field  type,  having  16  poles, 
and  generating  2,000  volts  and  576  amperes  per  phase,  when  run 
at  300  revolutions  per  minute. 


3800- 


3600- 

Hh 


135  K.W.    2  PHASE    GFNERATOR 
2400  V- 900   P.P.  M. — 60       P.  PS. 


8     10    12    14     16    18  20  22  24  26  28  30  32  34   36  3840 
ARMATURE    CURREMT 

Fig.  191.     Set  of  Curves  for  Two-Phase  135-Kw.  Inductor  Alternator 

Fig.  191  gives  a  number  of  curves  for  a  two-phase  135-kilowatt 
2,400-volt  60-cycle  inductor  alternator.  In  particular,  the  no-load 
saturation  curve  is  shown  giving  the  relation  of  the  field  current 
to  the  voltage  between  armature  terminals  of  one  phase.  This 
curve  shows  that  nearly  7  amperes  of  field  current  is  required  to 
give  rated  voltage,  namely  2,400,  at  no  load.  The  field  current 
required  to  give  rated  voltage  at  full  non-inductive  load  is  8.8  am- 


196 


ALTERNATING-CURRENT  MACHINERY 


peres,  as  is  explained  on  page  202.  From  the  saturation  curve  it  is 
evident  that  this  full-load  field  current  will  produce  about  2,625 
volts  at  no  load. 

Synchronous  Impedance  Curve.*  The  synchronous  impedance 
curve  shows  the  relation  between  armature  voltage  and  armature 
current,  the  armature  being  j>hort-circuited  so  that  the  only  condi- 
tion that  limits  the 
current  for  a  given  volt- 
age generated  is  the 
synchronous  impedance 
of  the  armature.  This 
is  materially  different 
from  the  impedance  of 
the  armature  when  the 
machine  is  standing 
still. 

The  connections  for 
this  test  are  similar  to 
those  for  the  saturation 
curve,  except  that  the 
voltmeter  (or  potential 
transformer)  connected 
to  the  armature  is  re- 
placed by  an  ammeter. 
If  the  current  is  be- 
yond the  capacity  of 
the  ammeters  at  hand, 
a  current  transformer 
may  be  connected  in 

place  of  the  ammeter,  and  the  ammeter  may  be  connected  to  its 
secondary. 

A  series  of  observations  is  to  be  taken  of  the  current  in  the 
armature,  with  the  latter  short-circuited  through  the  ammeter, 
for  different  field  currents,  commencing  at  a  very  low  value,  and 
increasing  the  field  current  by  successive  steps  until  the  armature 
current  has  reached  a  value  of  100  per  cent  above  its  rated  full- 
load  value.  The  last  few  readings  must  be  made  quickly  to  pre- 

*See  page  108. 


eoo 

/ 

1600 

, 

/ 

/ 

1400 

/ 

/ 

1200 

/ 

1000 

/ 

eoo 

/ 

/ 

eoo 

/ 

400 

/ 

Synch 

ronous 

/mpecti 

nee  D' 

*f> 

POO  / 

/ 

300 

o  k  \N  A 

for 
ffernah 

/• 

/ 

150 

300 

450 

600 

750 

900 

Amperes 

Fig.   192.      Curve   Showing    Synchronous   Impedance 
Drop  for  2,000-Kw.  Alternator 


ALTERNATING-CURRENT  MACHINERY  197 

vent  undue  heating  of  the  armature.  The  armature  winding  should 
be  at  approximately  normal  temperature  when  the  test  is  made. 
The  speed  should  be  kept  approximately  at  the  rated  speed  of  the 
machine.  It  is  not  as  essential  to  keep  the  speed  constantly  at  rated 
value  as  when  observations  are  being  made  for  the  determination 
of  the  saturation  curve.  The  observations  to  be  recorded  are: 
amperes  in  the  armature;  amperes  in  field;  volts  at  field  terminals;  and 


Fig.  192  shows  a  curve  giving  the  relation  between  electromo- 
tive force  induced  in  the  armature  and  the  current  in  the  armature 
when  short-circuited  through  an  ammeter.  This  figure  relates  to 
the  same  2,000-kilowatt  alternator  whose  saturation  curve  was 
given  in  Fig.  190.  The  electromotive  forces  plotted  in  this  figure 
are  not  observed  values,  but  the  field  excitations  required  to  pro- 
duce them  are  observed,  and  the  electromotive  forces  correspond- 
ing to  these  field  excitations  are  taken  from  the  saturation  curve. 

The  total  electromotive  force  induced  in  the  armature  for  a 
given  value  of  the  field  current  may  be  read  off  from  the  no-load 
saturation  curve  of  the  machine,  obtained  as  previously  described. 
A  curve  may  then  be  plotted  with  the  electromotive  force  induced 
in  the  armature  as  ordinates,  and  the  observed  armature  currents 
(on  short-circuit)  as  abscissas.  This  curve  is  sometimes  called 
the  synchronous  impedance  curve,  although  it  does  not  explicitly 
show  the  values  of  the  synchronous  impedance  of  the  armature. 
The  synchronous  impedance  of  the  armature  for  a  given  value  of 
armature  current  may,  however,  be  derived  from  this  curve  by  divid- 
ing the  total  electromotive  force  induced  in  the  armature  (ordinate) 
by  the  corresponding  value  of  the  short-circuited  armature  current 
(abscissa).  For  example,  the  synchronous  impedance  correspond- 
ing to  576  amperes  (which  is  the  full-load  current  per  phase  of  the 

machine)  is 

1,145  volts         n. 

=  1.99  ohms 

576  amperes 

Synchronous  impedance  is  used  as  a  basis  for  the  predeter- 
mination of  the  regulation  of  the  machine,  thereby  avoiding  the 
trouble  and  expense  of  an  actual  test  of  regulation  under  full  load. 
The  synchronous  impedance  of  an  alternator  armature  is  very 
nearly  equal  to  the  synchronous  reactance  of  the  armature,  inas- 


198 


ALTERNATING-CURRENT  MACHINERY 


much  as  the  armature  resistance  is  usually  small.  Moreover  the 
electromotive  force  required  to  overcome  synchronous  reactance  is 
very  nearly  equal  to  the  electromotive  force  required  to  overcome 
synchronous  impedance. 

Determination  of  Resistance  of  Armature.    In  the  case  of  a 
three-phase  armature,  the  resistance  per  phase  cannot  be  measured 

directly  between  collector  rings,  since 
there  are  two  phases  in  series  between 
collector  rings  in  a  Y-connected  arma- 
ture; while  in  case  of  a  A-connected 
armature,  two  phases  in  series  are  in 
parallel  with  the  third  phase  between 
any  two  collector  rings. 

The  resistance  per  phase   can  be 
measured  either  directly  by  Wheatstone 
Fig.  193.  Diagram  for  Armature  when  bridge    and    galvanometer,   or  by  the 

"fall   of  potential"   method.     In    the 

case  of  a  Y-connected  armature,  the  resistance  per  phase  is  one- 
half  the  resistance  between  terminals,  provided  that  the  resistance 
of  all  the  phases  are  alike.     In  case  the  resistances  of  the  phases  are 
unequal,  the  resistance  of  any  phase  may  be  deduced  as  follows: 
The  resistance  between  terminals  A  and  B,  Fig.  193,  is 


RAB  —  a  +  b 
The  resistance  between  terminals  B  and  C  is 

REC  =  6  +  c 
The  resistance  between  terminals  C  and  A  is 

RCA  =  c+'a 
Then 

CL  —  RAB  —  b 
b=  RBC—C 
C  =  RCA  —  a 

Substituting  (m)  in  (v)  we"obtain 

b  =  RBC —  RCA"}"  O> 
Substituting  (mi)  in  (iv)  we  obtain 


(n) 

(Hi) 

(«) 

(«) 

(n) 

(oil) 


ALTERNATING-CURRENT  I  MACHINERY 


199 


from  which 


Similarly  we  find 


Ci  ==  RAB  —  RBC  ~f~  RCA  —  & 
RAB  —  RBC  H~  RCA 


a  = 


RBC  —  RCA  +  RAB 


and 


RCA— 


R 


BC 


If  RAB  =  RBC  =  RCA,  then  a=  6  =  c  = 


R 


AB 


MB —    -Ll<BC —    J-^CA) 

~4 

For  a  A-connected  armature  with  equal ,  resistances  per  phase, 
the  resistance  per  phase  equals  |  times  the  resistance  between 
terminals.  The  general  expression  for 
the  resistance  of  any  phase  can  be  de- 
duced as  in  the  above  case  for  Y  con- 
nection. In  this  case  there  are  two 
circuits  between  A  and  B,  one  through 
the  phase  a,  and  the  other  through 
phases  b  and  c  in  series,  as  shown 
in  Fig.  194.  Remembering  that  the 
joint  resistance  of  two  (or  more)  circuits 
in  parallel  is  the  reciprocal  of  the  sum 
of  the  reciprocals  of  the  resistances  of  the  several  branches,  we  have 


Fig.   194.      Resistance  Diagram  for 
A-Connected  Armature 


RAR  — 


RBC  — 


RCA  = 


1 


— +— 

a        b-\-c 
1 


b    '  c+a 
1 

i  +  a+b 


From  these  three  equations  the  three  unknown  quantities  a,  6,  and 


200          ALTERNATING-CURRENT  MACHINERY 

c,   may  be  found  by  algebraic  elimination  as  in  the  case  of  the  Y 
connection. 

Regulation.*  The  saturation  and  synchronous  impedance 
curves  of  an  alternator  together  with  the  resistance  per  phase  of 
the  armature,  are  all  the  data  required  for  the  computation  of  the 
regulation  of  an  alternator.  The  direct  determination  of  regulation 
by  observation  would  be  as  follows: 

The  alternator  would  be  run  at  normal  rated  speed,  delivering  rated 
full-load  current  with  rated  full-load  electromotive  force  at  its  terminals. 
The  main  circuit  would  then  be  opened,  thus  reducing  the  current  output  to 
zero.  The  excitation  would  be  left  unchanged,  and  the  rise  of  terminal  electro- 
motive force  would  be  observed.  Then 

rise  of  terminal  electromotive  force 

regulation  in  per  cent  =  —         — — : — - — : — ; X  100 

rated  full-load  terminal  electromotive  force 

This  direct  determination  of  regulation  by  observation  is  not 
feasible  with  large  machines,  on  account  of  the  large  amount  of 
power  required.  In  all  practical  testing  the  regulation  may  be 
determined  indirectly  by  calculation  as  follows: 

When  the  alternator  is  operating  at  full  load,  the  total  electromotive 
force  induced  in  the  armature  exceeds  the  terminal  electromotive  force  by 
the  amount  lost  in  overcoming  armature  resistance,  and  by  the  amount  lost 
in  overcoming  the  synchronous  reactancef  of  the  armature. 

Let  E  be  the  total  induced  electromotive  force  in  the  armature  of  an 
alternator,  Et  the  terminal  electromotive  force  at  full  load,  IR  the  electro- 
motive force  lost  in  overcoming  armature  resistance,  and  X I  the  electromotive 
force  lost  in  overcoming  the  synchronous  reactance  of  the  armature.  The 
electromotive  force  required  to  produce  the  current  I  through  the  short-circuited 
armature  may  be  found  from  the  "synchronous  impedance  curve"  Fig.  192, 
by  taking  the  ordinate  corresponding  to  the  value  of  7  (abscissa) .  The  electro- 
motive force  so  found  is  nearly  equal  to  XI  on  account  of  the  relative  small- 
ness  of  armature  resistance,  and  especially  on  account  of  the  fact  that  IR  and 
XI  are  at  right  angles  to  each  other.  In  other  words,  the  synchronous  impe- 
dance is  in  most  practical  cases  approximately  equal  to  the  synchronous  re- 
actance. 

Therefore,  the  electromotive  force  found  from  the  synchronous  imped- 
ance curve  may  be  taken  as  the  value  of  XI. 

Knowing  the  electromotive  force  lost  in  overcoming  the  synchronous 
reactance,  we  can  find  from  the  saturation  curve  the  amount  of  field  excitation 
(abscissa)  required  to  produce  this  electromotive  force  (ordinate).  This 
field  excitation  is  represented  by  the  line  OB,  Fig.  195.  Take  from  the  eatura- 


*See  page  108. 
tSee  page  108. 


ALTERNATING-CURRENT  MACHINERY  201 

tion  curve  the  field  excitation  corresponding  to  rated  terminal  voltage  at  full 
load,  and  represent  it  by  the  line  OA,  Fig.  195.  Next  find  the  geometric  sum 
0(7,  of  OA  and  OB.  This  OC  represents  the  field  excitation  required  at  full 
load  to  produce  rated  terminal  electromotive  force.  The  electromotive  force 
produced  by  this  excitation  at  zero  load  may  be  taken  from  the  saturation 
curve.  The  difference  between  the  rated  electromotive  force  of  the  alternator 
and  the  electromotive  force  so  found,  is  the  rise  of  electromotive  force  from 
full  load  to  zero  load;  and  this  rise  divided  by  the  rated  electromotive  force 
and  multiplied  by  100  gives  the  regulation  of  the  alternator  in  per  cent. 

The  diagram,  Fig.  195,  applies  to  a  non-inductive  receiving  circuit. 
When  the  receiving  circuit  is  inductive,  the  angle  BOA,  Fig.  195,  should  be 
90  —  0,  in  which  6  is  the  angle  of  lag  of  the  current  behind  the  electromotive 
force  at  the  terminals  of  the  alternator.  The  cosine  of  this  angle  is  the  power 
factor  of  the  receiving  circuit. 

The  above  method  for  calculating  regulation  is  much  used  in 
practice,  and  is  recommended  by  the  Committee  on  Standardization 
of  the  American  Institute  of  Electrical  Engineers;  but  the  rule  is 
ambiguous  and  open  to  criticism.  Its  application  to  general  prac- 
tice may  be  shown  by  the  following : 

The  synchronous  impedance 
and  saturation  curves  shown  in 
Figs.  190  and  192  are  taken  from 
tests  upon  a  A-connected,  three- 

i  r»  t\f\r\  i  t  n         1       r>  nnn         lx  FiS-  195-     Vector  Diagram  for  Field 

phase,  2,000-KW.,  16-pOle,  2,000-VOlt,  Excitation  in  a  Non-inductive 

.    .          _    ,  .       .  ,  Receiving  Circuit 

revolving- field  alternator  having  a 

speed  of  300  r.  p.  m.  The  full-load  armature  current  is  576  amperes 
per  phase.  The  armature  resistance  per  phase  is  0.009239  ohms. 
Hence,  the  IR  drop  in  the  armature  =  (576  X  0.009239)  =  5.3  volts. 
Et  =  2,000;  Et+  IR  =  2,000+5.3  =  2005.3  volts. 

From  the  saturation  curve,  Fig.  190,  we  find  that  it  requires 
83.5  amperes  in  the  field  winding  to  generate  this  2,005.3  volts  at 
no  load.  This  represents  the  component  OA  in  Fig.  195.  From 
the  synchronous  impedance  curve,  Fig.  192,  we  find  that  1,137 
volts  in  the  armature  are  required  to  force  the  full-load  current  of 
576  amperes  per  phase  through  the  armature.  From  the  satura- 
tion curve,  we  find  that  1,137  volts  correspond  to  43  amperes  in 
the  field  winding.  The  component  OB  in  Fig.  195  is,  therefore, 
43  amperes.  The  full-load  field  current  OC  is 

OC  =  VOAZ+OBZ  =  l/432+83.52  =  93.8  amperes 
This  field  current  would  produce  at  zero  load  an  electromotive 


202  ALTERNATING-CURRENT  MACHINERY 

force  of  2,169  volts,  as  shown  by  the  saturation  curve.    Therefore, 
according  to  the  definition  of  regulation,  we  have 


Regulation  =  X  100  =  X  100  =  8.45  per  cent 


If  the  regulation  had  been  desired  at  a  power  factor  other 
than  unity  (for  instance,  at  85  per  cent  power  factor),  the  field  cur- 
rent OB,  Fig.  195,  would  not  be  taken  at  right  angles  to  OA  to  find 
the  resultant  field  current  OC  at  full  load,  but  the  angle  BOA  would 
be  decreased  by  the  angle  whose  cosine  is  0.85,  or  by  31.75°.  The 
resultant  field  current  OC  is  represented  as  before,  by  the  diagonal 
of  the  parallelogram;  but  it  is  no  longer  equal  to  the  square  root  of 
the  sum  of  the  squares  of  OA  and  OB.  The  angle  B  OA,  Fig.  194, 
now  is  58.25°  and,  therefore, 

OC2!=  OA2  +  052  +  2  OA  X  OB  cos  58.25° 
whence 

OC  =  112  amperes 

By  referring  to  the  saturation  curve,  we  find  that  a  full-load  field 
current  of  112  amperes  would  produce  2,375  volts  at  no  load.  Hence- 
the  regulation  at  85  per  cent  power  factor  is 


This  example  illustrates  the  general  fact  already  explained,  that 
the  electromotive  force  at  the  terminals  of  an  alternator  suffers 
a  greater  decrease  in  value  on  an  inductive  load  (power  factor  less 
than  unity)  than  on  a  non-inductive  load  (power  factor  unity)  for 
the  same  current  output. 

Regulation  Curve.  Fig.  191  shows  the  regulation  curves  of  a  two- 
phase,  135-kilowatt,  2,400-  volt,  60-cycle  inductor  alternator.  The 
lower  regulation  curve  shows  the  regulation  of  the  alternator  at 
100  per  cent  power  factor,  that  is,  on  non-inductive  load;  and  the 
upper  regulation  curve  shows  the  regulation  of  the  alternator  at 
70  per  cent  power  factor.  The  abscissa  of  a"  given  point  on  one  of 
these  regulation  curves  represents  a  given  current  output  per  phase 
of  the  machine;  and  the  ordinate  of  the  point  represents  the  voltage 
obtained  at  the  armature  terminals  when  this  armature  current 


ALTERNATING-CURRENT  MACHINERY  203 

is  reduced  to  zero,  the  field  current  being  kept  constant  at  that 
value  which  gives  the  rated  voltage  of  2,400  volts  with  the  given 
current  output  per  phase. 

For  example,  with  full-load  current  output,  namely  28.1  am- 
peres per  phase  and  100  per  cent  power  factor,  the  voltage  rises 
from  2,400  to  2,625  (the  ordinate  of  the  regulation  curve  for  100 
per  cent  power  factor  corresponding  to  the  abscissa  representing 
28.1  amperes  of  armature  current)  when  the  load  is  thrown  off  (arma- 
ture current  reduced  to  zero),  the  field  current  remaining  unchanged. 
From  these  data  the  full  load  regulation  of  the  machine  on  100  per 
cent  power  factor  is  found  to  be 

225 
2400  X  100  =  9.4  per  cent 

With  half-load  current  output,  namely  14.05  amperes  per 
phase,  [and  70  per  cent  power  factor,  [the  voltage  rises  from  2,400 
to  2,650  when  the  load  is  thrown  off,  the  field  current  remaining 
unchanged.  From  these  data  the  half-load  regulation  of  the  machine 
at  70  per  cent  power  factor  is  found  to  be 

250 
2^X100  =10.4  per  cent 

These  curves  show  that  the  regulation  of  the  machine  is  higher  on 
inductive  loads  than  on  non-inductive  loads. 

American  Institute  Rules.  The  following  are  the  recommenda- 
tions of  the  American  Institute  of  Electrical  Engineers  concerning 
the  regulation  of  electrical  apparatus  and  prime  movers. 

DEFINITIONS 

187  REGULATION.     The  regulation  of  a  machine  or  apparatus  in  regard  to 
some  characteristic  quantity  (such  as  terminal  voltage,  current,  or  speed) 
is  the  ratio  of  the  deviation  of  that  quantity  from  its  normal  value  at  rated 
load  to  the  normal  rated  load  value.     The  term  "regulation,"  therefore, 
•has  the  same  meaning  as  the  term  "inherent  regulation,"  occasionally 
used. 

188  CONSTANT  STANDARD.     If  the  characteristic  quantity  is  intended  to  re- 
main constant  (e.g.,  constant  voltage,  constant  speed,  etc.)  between  rated 
load  and  no  load,  the  regulation  is  the  ratio  of  the  maximum  variation 
from  the  rated  load  value  to  the  no-load  value. 

189  VARYING  STANDARD.     If  the  characteristic  quantity  is  intended  to  vary 
in  a  definite  manner  between  rated  load  and  no  load,  the  regulation  is 


204  ALTERNATING-CURRENT  MACHINERY 

the  ratio  of  the  maximum  variation  from  the  specified  condition  to  the 
normal  rated-load  value. 

190  NOTE  (a)     If  the  law  of  the  variation  (in  voltage,  current,  speed,  etc.) 
between  rated  load  and  no  load  is  not  specified,  it  should  be  assumed  to 
be  a  simple  linear  relation;  i.  e.,  one  undergoing  uniform  variation  between 
rated  load  and  no  load. 

191  NOTE  (b)     The  regulation  of  an  apparatus  may,  therefore,  differ  accord- 
ing to  its  qualification  for  use.     Thus,  the  regulation  of  a  compound- 
wound  generator  specified  as  a  constant-potential  generator,  will  be  differ- 
ent from  that  which  it  possesses  when  specified  as  an  over-compounded 
generator. 

192  In  CONSTANT-POTENTIAL  MACHINES,  the  regulation  is  the  ratio  of  the 
maximum  difference  of  terminal  voltage  from  the  rated-load  value  (occur- 
ring within  the  range  from  rated  load  to  open  circuit)  to  the  rated  load 
terminal  voltage. 

193  In  CONSTANT-CURRENT  MACHINES,  the  regulation  is  the  ratio  of  the 
maximum    difference  of    current  from  the  rated-load  value   (occurring 
within  the  range  from  rated-load  to  short-circuit,  or  minimum  limit  of 
operation),  to  the  rated-load  current. 

194  In  CONSTANT-POWER  APPARATUS,  the  regulation  is  the  ratio  of  maxi- 
mum difference  of  power  from  the  rated-load  value  (occurring  within  the 
range  of  operation  specfied)  to  the  rated  power.  • 

195  In  CONSTANT-SPEED  DIRECT-CURRENT  MOTORS  and    INDUCTION  MO- 
TORS, the  regulation  is  the  ratio  of  the  maximum  variation  of  speed  from 
its  rated  load  value  (occurring  within  the  range  from  rated  load  to  no  load) 
to  the  rated-load  speed. 

196  The  regulation  of  an  induction  motor  is,  therefore,  not  identical  with 
the  slip  of  the  motor,  which  is  the  ratio  of  the  drop  in  speed  from  syn- 
chronism, to  the  synchronous  speed. 

197  In  CONSTANT-POTENTIAL  TRANSFORMERS,  the  regulation  is  the  ratio  of 
the  rise  of  secondary  terminal  voltage  from  rated  non-inductive  load  to 
no-load  (at  constant  primary  impressed  terminal  voltage)  to  the  secondary 
terminal  voltage  at  rated  load. 

198  In  OVER-COMPOUNDED  MACHINES,  the  regulation  is  the  ratio  of  the 
maximum  difference  in  voltage  from  a  straight  line  connecting  the  no-load 
and  rated-load  values  of  terminal  voltage  as  function  of  the  load  current, 
to  the  rated-load  terminal  voltage. 

199  In  CONVERTERS,    DYNAMOTORS,  MOTOR-GENERATORS  and  FREQUENCY 
CONVERTERS,  the  regulation  is  the  ratio  of  the  maximum  difference  of 
terminal  voltage  at  the  output  side  from  the  rated-load  voltage,  to  the 
rated-load  voltage  on  the  output  side. 

200  In  TRANSMISSION  LINES,  FEEDERS,  ETC.,  the  regulation  is  the  ratio  of 
the  maximum  voltage  difference  at  the  receiving  end,  between  rated  non- 
inductive  load  and  no  load  to  the  rated-load  voltage  at  the  receiving 
end  (with  constant  voltage  impressed  upon  the  sending  end). 

201  In  STEAM  ENGINES,  the  regulation  is  the  ratio  of  the  maximum  varia- 
tion of  speed  in  passing  slowly  from  rated-load  to  no-load  (with  constant 
steam  pressure  at  the  throttle)  to  the  rated-load  speed. 


ALTERNATING-CURRENT  MACHINERY  205 

202  In  a  HYDRAULIC  TURBINE  or  OTHER  WATER-MOTOR,  the  regulation  is 
the  ratio  of  the  maximum  variation  of  speed  in  passing  slowly  from  rated- 
load  to  no-load  (at  constant  head  of  water;  i.  e.,  at  constant  difference  or 
level  between  tail  race  and  head  race),  to  the  rated  load  speed. 

203  In  a  GENERATOR-UNIT,  consisting  of  a  generator  united  with  a  prime- 
mover,  the  regulation  should  be  determined  at  constant  conditions  of  the 
prime-mover;  i.  e.,  constant  steam  pressure,  head,  etc.     It  includes  the 
inherent  speed  variations  of  the  prime-mover.     For  this  reason  the  regu- 
lation of  a  generator-unit  is  to  be  distinguished  from  the  regulation  of 
either  the  prime-mover,  or  of  the  generator  contained  in  it,  when  taken 
separately. 

CONDITIONS  FOR  AND  TESTS  OF  REGULATION 

204  SPEED.     The  regulation  of  generators  is  to  be  determined  at  constant 
speed,   and  of  alternating  apparatus  at  constant  impressed  frequency. 

205  NON-INDUCTIVE    LOAD.     In    apparatus    generating,    transforming,    or 
transmitting  alternating  currents,  regulation  should  be  understood  to  refer 
to  non-inductive  load,  that  is,  to  a  load  in  which   the  current  is  in  phase 
with  the  e.  m.f.  at  the  output   side  of  the  apparatus,  except  where  ex- 
pressly specified  otherwise. 

206  WAVE  FORM.     In  alternating  apparatus  receiving  electric  power,  regu- 
lation should  refer  to  a  sine  wave  of  e.  m.f.,  except  where  expressly  speci- 
fied otherwise. 

207  EXCITATION.    In  commutating  machines,  rectifying  machines,  and  syn- 
chronous machines,  such  as  direct-current  generators  and  motors,  alter- 
nating-current and  polyphase  generators,  the  regulation  is  to  be  deter- 
mined under  the  following  conditions: 

(1)  At  constant  excitation  in  separately  excited  fields. 

(2)  With  constant  resistance  in  shunt-field  circuits,  and 

(3)  With  constant  resistance  shunting  series-field  circuits;  i.  e.,  the 
field  adjustment  should  remain  constant,  and  should  be  so  chosen  as  to 
give  the  required  full-load  voltage  at  full-load  current. 

208  IMPEDANCE  RATIO.    In  alternating-current  apparatus,  in  addition  to  the 
non-inductive  regulation,  the  impedance  ratio  of  the  apparatus  should  be 
specified;  i.  e.,  the  ratio  of  the  voltage  consumed  by  the  total  internal  im- 
pedance of  the  apparatus  at  full-load  current,  to  its  rated  full-load  voltage. 
As  far  as  possible,  a  sinusoidal  current  should  be  used. 

209  COMPUTATION  OF  REGULATION.     When  in  synchronous  machines,  the 
regulation  is  computed  from  the  terminal  voltage  and  impedance  voltage, 
the  exciting  ampere-turns  corresponding  to  terminal  voltage  plus  armature- 
resistance-drop,  and  the  ampere-turns  at  short-circuit  corresponding  to 
the  armature-impedance-drop,  should  be  combined  vectorially  to  obtain 
the  resultant  ampere-turns,  and  the  corresponding  internal  e.  m.  f.  should 
be  taken  from  the  saturation  curve.    • 

Heat  Test.  The  heat  test  is  made  by  running  the  generator 
under  full-load  conditions  until  a  constant  temperature  has  been 
reached.  When  this  condition  has  been  attained  the  machine  is 


206  ALTERNATING-CURRENT  MACHINERY 

shut  down,  and  the  temperature  of  the  various  parts  is  taken  by 
thermometers  placed  against  the  heated  surfaces.  The  resistances 
of  the  armature  and  field  windings  are  also  measured  while  hot. 
By  comparing  these  "hot"  resistances  with  the  same  resistances 
previously  measured  at  room  temperature,  the  temperature  rise  of 
the  armature  and  field  coils  can  be  computed.  The  method  recom- 
mended by  the  Standardization  Committee  of  the  American  Institute 
of  Electrical  Engineers  for  making  these  computations  is  as  follows: 

The  fundamental  relation  between  the  increase  of  resistance  in  copper 
and  the  rise  of  temperature  may  be  taken  as 

Rt=R0  (1+0.00420 

where  RQ  is  the  resistance  of  the  copper  conductor  at  0°C.  and  Rt  is  the  cor- 
responding resistance  at  t°  C.  This  is  equivalent  to  taking  a  temperature  co- 
efficent  of  0.42  per  cent  per  degree  C.  temperature  rise  above  0°  C.  For 
initial  temperatures  other  than  0°C.,  a  similar  formula  may  be  used  sub- 
stituting the  proper  coefficient  corresponding  to  the  actual  initial  temperature. 
The  formula  thus  becomes  at  25°  C. 

p,,  0.3801  r 

**r"*(,+  -~T5o~~ 

where  Rt  is  the  initial  resistance  at  25  °C.  Ri  +  r  the  final  resistance,  and 
r  the  temperature  rise  above  25  °C. 

In  order  to  find  the  temperature  rise  in  degrees  centigrade  from  the  initial 
resistance  R{  at  the  initial  temperature  i°C.  and  the  final  resistance  #,  +  „ 
we  may  use  the  formula 

I") 

r  =  (238.1  +  i)   (  -  ^  —  1)   degrees  C. 
Ri 

Example.  The  "cold"  resistance  of  the  armature  of  a  generator  is  meas- 
ured and  found  to  be  0.046  ohm  at  a  temperature  of  20.5°C.  After  a  heat  run 
under  full  load  the  resistance  is  found  to  be  0.052  ohm.  What  is  the  rise  in 
temperature? 

In  this  case  the  initial  temperature  i  is  20.5°C.,  the  final  resistance  R{+  r 
is  0.052  ohm,  and  the  initial  resistance  Rt  is  0.046  ohm.  The  temperature  rise 
is,  therefore, 


r-  (238.1+20.5)   (  -  1)  =  33.72  centigrade  degrees 

0.046 

which  means  that  the  final  temperature  of  the  armature  is 
i  +  r  =  20.5  +  33.72  =  54.22  °C 

The  temperatures  of  the  following  generator  parts  are  usually  re- 
corded : 


ALTERNATING-CURRENT  MACHINERY  207 

Armature  coils  Field  coils 

Armature  laminations  Pole  tip  (leading) 

Armature  ventilating  ducts  Pole  tip  (trailing) 

Frame  Field  yoke 

Bearings  Room 

Small  generators  are  usually  run  at  full  load,  delivering  their 
output  to  water  rheostats.  During  the  heat  run,  thermometers, 
placed  on  different  parts  of  the  machine,  are  read  regularly^If 
the  generator  is  of  the  revolving-armature  type,  a  thermometer  is 
placed  against  the  field  winding,  and  another  thermometer  is  so 
placed  that  the  hot  air  issuing  from  the  ventilating  ducts  will  come 
in  contact  with  it.  If  the  generator  be  of  the  revolving-field  type, 
thermometers  are  placed  against  the  armature  coil,  on  the  armature 
laminations,  and  in  the  ventilating  ducts.  When  these  thermometers 
do  not  show  an  increasing  temperature,  and  the  resistance  of  the 
field,  as  determined  from  the  field  ammeter  and  voltmeter,  has 
become  constant,  the  machine  is  considered  as  having  attained 
its  ultimate  temperature,  and  is  shut  down  for  the  application  of 
thermometers,  which  should  be  ready  for  immediate  application,  as 
the  machine  cools  rapidly.  The  time  taken  by  a  machine  to  reach 
constant  temperature  varies  from  3  to  4  hours  in  the  case  of  a  small 
machine,  and  from  12  to  18  hours  for  very  large  ones. 

The  following  maximum  values  of  temperature  elevation  have 
been  recommended  by  the  Standardization  Committee  of  the  Ameri- 
can Institute  of  Electrical  Engineers: 

Field  and  armature,  50 °C.  by  resistance 

Commutator  and  brushes,  55°C.  by  thermometer 

Collector  rings,  65°C.  by  thermometer 

Bearings  and  other  parts  of  machine,  40°C.  by  thermometer 

The  rise  of  temperature  should  be  referred  to  the  standard 
conditions  of  a  room  temperature  of  25°C.,  a  barometric  pressure 
of  760  mm.,  and  normal  conditions  of  ventilation;  that  is,  the  ap- 
paratus under  test  should  neither  be  exposed  to  draft  nor  enclosed, 
except  where  expressly  specified. 

If  the  room  temperature  during  the  test  differs  from  25°C., 
the  observed  rise  of  temperature  should  be  corrected  by  J  per  cent 
for  each  degree  C.  Thus,  with  a  room  temperature  of  35°C.,  the 
observed  rise  of  temperature  has  to  be  decreased  by  5  per  cent; 
and  with  a  room  temperature  of  15°C.,  the  observed  rise  of  tern- 


208  ALTERNATING-CURRENT  MACHINERY 

perature  has  to  be  increased  by  5  per  cent.  The  thermometer  in- 
dicating the  room  temperature  should  be  screened  from  thermal 
radiation  emitted  by  heated  bodies  or  from  drafts  of  air.  When  it  is 
impracticable  to  secure  normal  conditions  of  ventilation  on  account 
of  an  adjacent  engine,  or  other  sources  of  heat,  the  thermometer 
for  measuring  the  air  temperature  should  be  placed  so  as  to  fairly 
indicate  the  temperature  which  the  machine  would  have  if  it  were 
idle,  in  order  that  the  rise  of  temperature  determined  shall  be  that 
caused  by  the  operation  of  the  machine. 

_  The  temperature  should  be  measured  after  a  run  of  sufficient 
duration  to  reach  practical  constancy.  This  is  usually  from  6  to 
18  hours,  according  to  the  size  and  construction  of  the  apparatus. 
It  is  permissible,  however,  to  shorten  the  time  of  the  test  by  run- 
ning a  lesser  time  on  an  overload  in  current  and  voltage,  then  reduc- 
ing the  load  to  normal,  and  maintaining  it  thus  until  the  temperature 
has  become  constant. 

In  making  a  heat  test  of  a  large  alternator  it  is  customary  to 
imitate  full-load  conditions,  electrically  and  magnetically,  so  as  to 
produce  all  the  heating  effects  that  would  occur  under  actual  full 
load,  but  without  taking  any  actual  electrical  power  from  the  alter- 
nator. To  be  able  to  do  this  is  of  great  advantage  from  two  stand- 
points: first,  that  of  convenience;  and  second,  that  of  economy.  To 
accomplish  this  desirable  result,  the  generator  is  usually  run  on  short- 
circuit  with  a  number  of  the  field  spools  connected  in  opposition 
to  (or  "bucked"  against)  the  remainder;  or,  in  other  words,  with  the 
effective  number  of  poles  reduced. 

To  determine  the  proper  number  of  field-magnet  spools  to  be 
connected  in  opposition,  we  proceed  in  the  following  manner: 

It  was  previously  explained  that  from  the  saturation  and  syn- 
chronous impedance  curves  of  a  generator,  we  could  determine  the 
field  current  required  to  produce  rated  voltage  at  full  load.  In 
the  case  of  the  2,000-kw.  generator,  of  which  the  curves  are  given 
in  Figs.  190  and  192,  the  normal  full-load  field  current  was  found  to 
be  93.8  amperes.  This  field  current  would,  if  the  armature  were 
short-circuited,  produce,  according  to  Fig.  192,  an  armature  cur- 
rent of  more  than  1,050  amperes,  which  is  greatly  in  excess  of  rated 
full-load  current.  If  the  field  current  were  reduced  to  43  amperes, 
the  armature  current  would  be  normal;  but  the  field  current  and, 


ALTERNATING-CURRENT  MACHINERY  209 

therefore,  the  magnetic  density  in  the  field  magnet  and  the  arma- 
ture core,  and  the  consequent  iron  losses  in  the  armature,  would 
be  very  much  below  normal. 

We  may  reduce  the  excessive  current  in  the  short-circuited 
armature  to  its  normal  full-load  value  by  reducing  the  effective 
number  of  field  poles  in  the  ratio  of  93.8  to  43  instead  of  by  reducing 
the  field  current  in  this  ratio.  The  alternator  has  16  field  poles, 
and  to  reduce  this  number  in  the  ratio  of  93.8  to  43,  would  give 

16  X  43 

—  =  7.34  poles;  but  inasmuch  as  there  must  be  an  integral 

9o.o 

and  even  number  of  field  poles,  the  closest  approximation  to  the 
desired  number  is  8  effective  poles.  If,  therefore,  we  reverse  the 
connections  of  any  four  (adjacent)  field  coils,  the  four  poles  pro- 
duced by  these  reversed  field  coils  will  be  reversed  in  polarity; 
and  the  electromotive  forces  induced  in  the  armature  conductors 
under  these  reversed  poles  will  be  reversed,  and  will  balance  the 
electromotive  forces  induced  in  the  armature  conductors  under 
four  of  the  unreversed  field  poles,  so  that  the  electromotive  forces 
induced  in  the  armature  conductors  under  the  remaining  eight 
poles  only,  will  be  effective  in  producing  current  in  the  armature, 
that  is,  only  eight  field  poles  will  be  effective. 

If,  under  these  conditions,  we  short-circuit  the  armature  of 
the  alternator,  it  will  take  approximately  the  full-load  field  cur- 
rent of  93.8  amperes  to  produce  full-load  current  in  the  armature, 
and  the  heat  test  can  then  be  made  under  full-load  conditions, 
namely,  full-load  armature  current,  full-load  field  current,  and 
full-load  iron  losses,  while  the  machine  is  running  on  short-circuit 
and,  therefore,  delivering  no  power.  Only  the  power  represented 
by  the  losses  occurring  in  the  machine  need  be  supplied  to  drive  it. 
This  is  a  method  often  used  in  practice.  If  the  normal  field  current 
does  not  give  normal  armature  current,  after  the  spools  are  con- 
nected in  opposition  as  described,  the  effect  of  a  slight  excess  of 
armature  current  may  be  approximately  balanced  by  a  slight  de- 
ficiency of  field  current,  or  vice  versd. 

Core  Loss  and  Friction  Test.  For  this  test  the  generator  is 
driven  by  a  motor  at  normal  speed  on  open  circuit.  The  power 
required  to  drive  it,  its  field  current  being  zero,  is  measured.  The 
power  so  measured  is  the  power  lost  in  friction  and  windage.  Then 


210 


ALTERNATING-CURRENT  MACHINERY 


•2,000  K  W. 


at  no  L  pod 


the  field  current  is  increased,  step  by  step,  until  the  terminal  electro- 
motive force  has  increased  to  from  25  to  50  per  cent  above  its  normal 
rated  full-load  value;  the  terminal  voltage  is  observed  at  each  step, 
the  armature  being  on  open  circuit  (main  switch  open).  The  power 
required  to  drive  the  machine  at  each  of  these  observed  voltages 
is  determined.  This  power  is  used  to  supply  friction  and  windage 
loss  and  iron  (or  core)  loss  at  the  observed  voltage. 

The  difference,  therefore,  between  the  power  required  to  drive 
the  machine  at  a  given  voltage  on  open  circuit,  and  the  amount 
of  power  required  to  drive  the  machine  without  field  excitation,  that 
is,  to  supply  the  friction  and  windage  loss,  represents  the  iron  loss, 
or  core  loss,  at  the  given  voltage.  Fig.  196  shows  the  no-load 
core-loss  curve  for  the  2,000-kw.  alternator  of  which  the  saturation 
curve  and  the  synchronous  impedance  curve  are  given  in  Figs.  190 

and  192,  respectively. 

The  most  convenient 
way  to  measure  the  power 
delivered  to  the  alternator 
is  to  drive  it  by  a  motor 
and  measure  the  electrical 
input  to  the  motor.  If  we 
determine  the  losses  occur- 
ring in  the  motor  the  dif- 
ference between  the  input 
to  the  motor  and  these 
losses  is  the  power  deliv- 
ered to  the  alternator.  Fig. 
197  shows  a  diagram  of 

complete  connections  for  the  carrying  out  of  the  core  loss  test. 
A  is  the  alternator  under  test.  Its  voltage  is  measured  by  the 
voltmeter  V\  connected  to  the  secondary  of  the  potential  trans- 
former T.  Its  field  current,  supplied  by  the  exciter  E,  is  measured 
by  the  ammeter  A\  and  controlled  by  the  resistance  RE  in  the 
field  of  the  exciter.  The  power  is  supplied  to  the  motor  M  by 
the  direct-current  generator  G.  The  power  is  measured  by  the 
ammeter  A2  and  the  voltmeter  F2.  The  motor  M  is  separately 
excited,  its  field  current  being  measured  by  the  ammeter  A3.  This 
field  current  is  kept  constant  by  adjusting  the  rheostat  RF. 


Full 


900 


200 


600 


2000 


Fig.:i96. 


Volts    in  Armature 

No-Load  Core-Loss  Curve  for  2,000-Kw. 
Alternator 


ALTERNATING-CURRENT  MACHINERY 


211 


As  the  field  current  and  the  voltage  of  the  generator  under  test 
are  increased,  the  load  on  the  driving  motor  M  is  increased  also, 
since  the  hysteresis  and  eddy  current  loss  in  the  generator,  which 
must  be  supplied  by  the  motor ,  are  increased.  As  the  load  on  the 
motor  increases,  it  will  slow  down  if  its  field  current  and  the  voltage 
between  its  brushes  remain  constant.  However,  the  alternator 
to  be  tested  must  be  run  at  constant  speed,  and  its  speed  is  con- 
trolled by  varying  the  voltage  at  the  motor  terminals  by  means~of 
the  field  rheostat  R0  of  the  direct-current  generator  G.  In  the 
figure  the  field  rheostat  R0  of  the  generator  G,  is  shown  beside 
the  alternator  A.  It  is  placed  at  this  point  for  convenience,  so  that 
the  observer  stationed  at  the  alternator  may  readily  keep  the  speed 
under  control. 


To  oufs/de  source 
of  c/irecf  current 

Fig.   197.     Diagram  of  Connections  for  Core-Loss  Test  of  Alternators 

The  input  to  the  motor  armature  in  watts  is  equal  to  the  pro- 
duct of  the  readings  V%  and  A^  Part  of  this  input  is  consumed 
in  supplying  the  various  losses  in  the  motor  armature,  including 
friction  and  windage;  and  the  remainder  is  converted  into  useful 
mechanical  power,  and  is  transmitted  by  the  belt  (not  shown  in 
Fig.  197)  to  the  pulley  of  the  alternator  A,  which  is  being  tested. 
In  other  words,  the  watts  input  to  A  is  equal  to  the  input  to  the 
armature  of  M  minus  the  various  losses  in  the  armature  of  M^  The 
losses  in  the  armature  of  M  consist  of  hysteresis,  eddy  current, 


212  ALTERNATING-CURRENT  MACHINERY 

friction,  windage,  and  PR  losses.  Since  the  alternator  is  to  be  run 
at  constant  speed,  the  motor  will  also  run  at  constant  speed  if  there 
is  no  belt  slip,  which  should  be  the  case;  and  since  the  motor  runs  at 
constant  speed  and  at  a  constant  field  excitation,  all  the  losses  in  the 
motor  armature  will  be  constant  in  value,  whatever  its  load,  except 
the  PR  loss.  This  PR  loss  varies  because  the  current  I  varies  for 
different  loads  on  the  motor.  If  the  speed  of  the  motor  were  con- 
trolled by  varying  its  field  current,  instead  of  varying  the  voltage 
applied  at  its  terminals,  the  above-mentioned  losses  in  the  motor 
armature  would  not  remain  constant;  hence  the  latter  method  is 
adopted. 

If  the  motor  be  run  free,  i.  e.,  with  belt  off,  the  output  of  the 
motor  will  be  zero  and,  therefore,  the  power  input  must  be  used  up 
entirely  in  supplying  the  losses.  Let  E  be  the  voltage  at  motor 
brushes,  and  7  the  current  in  the  motor  armature  when  it  is  run- 
ning free  (or  unloaded),  then  El—  stray  poweT-\-PR,  where  "stray 
power"  equals  the  sum  of  all  the  constant  losses  in  the  motor  arma- 
ture, i.e.,  hysteresis,  eddy  current,  friction,  and  windage  losses. 
Hence,  stray  poweT—EI—PR. 

The  mechanical  or  useful  output  of  the  motor  at  a  given  load, 
the  voltage  applied  to  its  brushes  being  EI,  and  its  armature  current 

being  /i,  is 

output  =  EJi  —  I\Ri  —  stray  power 

That  is,  the  mechanical  output  is  equal  to  the  total  electrical  input 
minus  the  total  losses. 

The  stray  power  can  be  determined  once  for  all  by  running 
the  motor  free,  or  at  no  load,  at  the  speed  and  with  the  field  current 
used  in  the  test  when  E  and  I  were  observed.  Thus,  we  may  deter- 
mine the  power  required  to  drive  A  (the  machine  to  be  tested) 
with  zero  field  excitation  of  A,  in  which  case  we  obtain  its  friction 
and  windage  losses;  and  we  may  determine  the  power  required  to 
drive  A  at  any  given  field  excitation  and  voltage,  in  which  case 
we  obtain  the  sum  of  the  friction,  windage,  and  core  (or  iron)  losses. 

In  carrying  out  these  tests,  therefore,  the  following  order  is 
found  most  convenient: 

(1)  The  motor  is  made  to  drive  the  alternator  with  zero  field  current 
in  the  latter;  and  a  number  of  observations  are  taken,  as  indicated  in  the  tabular 
arrangement  below.  This  is  to  determine  the  friction  and  windage  loss  of  the 
alternator. 


Motors 


ALTERNATING-CURRENT  MACHINERY  213 

(2)  The  alternator  is  excited  to  produce  a  series  of  values  of  terminal 
voltage,  for  each  of  which  a  similar  set  of  observations  is  taken;  and  so  on,  until 
the  voltage  of  the  alternator  has  been  increased  to  about  25  per  cent  above 
normal  rated  voltage. 

(3)  The  motor  is  now  shut  down  and  its  belt  thrown  off,  and  it  is  then 
run  unloaded  in  order  to  determine  its  own  stray-power  losses. 

(4)  Finally  the  motor  is  shut  down,  and  the  resistance  of  its  armature 
is  measured. 

For  each  set  of  readings  the  following  observations  should  be  recorded: 

Volts  at  brushe  (  Volts  at  terminals  ' 

Amperes  in  armature  Alternator  I  Amperes  in  field 
Amperes  in  field  ( Speed 

Speed 

The  amperes  in  the  field  of  the  motor,  and  the  speed  of  the  motor  and 
generator  are  recorded  merely  to  show  whether  they  have  been  kept  constant 
during  the  test.  For  the  best  results  the  motor  should  have  a  rated  capacity 
of  from  15  to  20  per  cent  of  the  rated  capacity  of  the  machine  to  be  tested. 

Example.  In  a  core-loss  test  of  the  2,000-kw.  alternator  above 
referred  to,  the  following  observations  were  taken:  With  zero  field  excitation 
of  the  alternator,  the  voltage  applied  to  the  motor  armature  and  the  current 
in  the  motor  armature  were  510  volts  and  48  amperes,  respectively.  When 
the  alternator  was  excited  to  give  2,070  volts  between  its  terminals,  the  volts 
applied  to,  and  the  current  in,  the  motor  armature  were  511  volts  and  117 
amperes,  respectively.  With  the  motor  running  unloaded  rthe  voltage  applied 
to,  and  the  current  in,  the  motor  armature  were  509  volts  and  11.5  amperes, 
respectively.  The  resistance  of  the  armature  of  the  motor  was  0.056  ohm. 
Find: 

(a)  The  stray-power  loss  of  the  driving  motor,  from  the  readings  taken 
above  when  the  motor  was  running  unloaded. 

Solution.  Motor  input  was  509  volts  X  11.5  amperes  =  5,853  watts. 
But  by  definition,  stray  power  —  El  — PR.  Hence 

stray  power  =  5,853  -  0.056  X  (11.5) 2  =  5,846  watts 

(b)  The  friction  and  windage  loss  of  the  alternator. 

Solution.  Power  supplied  to  motor  when  driving  alternator  at  full 
speed  with  zero  field  excitation  was  510  volts X  48  amperes  =  24, 480  watts; 
and  the  useful  output  of  the  motor  or  the  power  used  to  drive  the  alternator 
was  the  power  input  to  the  motor  minus  stray-power  loss  and  minus  PR  loss 
in  its  armature.  Therefore 

friction  and  windage  loss  =  24,480 -5,846- [(48) 2  X  0.056]  =  18,505  watts 

(c)  The  core  (or  iron)  loss  of  the  alternator. 

Solution.  Power  supplied  to  motor  when  driving  the  alternator  at 
full  speed,  field  excited  to  give  2,070  volts  between  alternator  terminals, 
was  511  volts  X 117  amperes  =  59, 787  watts.  The  useful  output  of  the  motor 
in  this  case  was  59,787  watts-5,846  watts -[(117)2X  0.056]  =  53,175  watts; 
and  this  is  equal  to  the  sum  of  friction  and  windage  loss  and  iron  loss.  Therefore 

core  loss  of  alternator  =  53,175  -  18,505  =  34,670  watts 
I 


214  ALTERNATING-CURRENT  MACHINERY 

The  core  loss  of  the  alternator  was  calculated  for  each  value 
of  the  field  excitation  of  the  alternator  in  a  manner  similar  to  the 
above.  These  various  core  losses  are  plotted  as  ordinates  of  the 
curve  in  Fig.  196,  the  abscissas  being  the  electromotive  forces  be- 
tween alternator  terminals  corresponding  to  the  various  field  ex- 
citations. This  curve  shows  that  at  the  rated  full-load  voltage,  namely, 
2,000  volts,  the  core  loss  of  the  alternator  is  30.5  kilowatts  or  1.525 
per  cent  of  the  rated  full-load  output  of  the  alternator. 

Calculation  of  Efficiency.  2,000-Kw.  Alternator.  Since  the 
efficiency  of  an  alternator  is  the  ratio  of  the  output  to  the  output 
plus  the  losses,  as  explained  on  page  181,  we  may  now  calculate  the 
efficiency  of  the  alternator,  having  made  all  necessary  tests.  In 
the  case  of  the  2,000-kw.  alternator  referred  to  above,  for  which 
the  curves  have  been  given,  the  following  losses  occur: 

(1)  Friction  and  windage  loss.    The  loss  was  found  under  (b) 
in  the  example  just  given  to  be  18,505  watts. 

(2)  PR  loss  in  armature  at  full  load.    The  resistance  of  each 
phase  of  the  armature  winding  of  the  given  alternator  was  measured 
and  found  to  be  0.00924  ohm,  and  the  full-load  rated  current  is  576 
amperes  per  phase.    Therefore,  the  PR  loss  per  phase  is 

(576)2  X  0.00924  =  3,065  watts 
Hence,  the  total  PR  loss  for  all  three  phases  is 

3  X  3,065  watts  =  9,195  watts 

(3)  PR  loss  in  the  field.    The  resistance  of  the  field  winding 
of  the  given  alternator  is  0.966  ohm,  and  the  full-load  field  current 
is  93.8  amperes.    Hence,  PR  loss  in  the  field  is  8,500  watts. 

(4)  Core  loss.    The  core  loss  on  open  circuit  at  full-load  excita- 
tion was  found  to  be  30,500  watts.    This  is  somewhat  less  than  the 
core  loss  would  be  with  full-load  excitation  and  with  full-load  arma- 
ture current. 

Therefore,  the  sum  of  all  the  losses  is 

Friction  and  windage  loss  18,505 

PR  loss  in  armature  9,195 

PR  loss  in  the  field  8,500 

Core  loss  .  30,500 

Total  loss  -  66,700  watts 


ALTERNATING-CURRENT  MACHINERY  215 

The  rated  full-load  output  being  2,000  kilowatts,  we  have 

2,000,000 
efficiency  at  full  load  =  2  WQQ  +  66?7QQ  =  96.8  per  cent 


If  the  efficiency  at  any  fractional  part  of  full  load  is  desired, 
it  may  be  calculated  as  follows: 

J  rated  full-load  output 


^  rated  full-load  output  +  losses  at  half  load 

(1)  Friction  and  windage  loss  is  approximately  the  same  at 
half  load  as  at  full  load. 

(2)  PR  loss  in  the  armature  is  one-fourth  as  great  as  at  full 
load  since  the  armature  current  is  half  as  great. 

(3)  PR  loss  in  the  field  is  slightly  less  at  half  load  than  at 
full  load,  inasmuch  as  field  current  is  slightly  less. 

(4)  Core  loss  at  half  load  is  slightly  less  than  core  loss  at  full 
load. 

135-Kw.  Inductor  Alternator.  Fig.  191  shows  the  efficiency  curve 
of  a  two-phase,  135-kilowatt,  2,400- volt,  60-cycle  inductor  alternator. 
The  ordinates  of  this  efficiency  curve  represent  the  efficiencies  of  this 
alternator  corresponding  to  different  current  outputs  per  phase, 
the  latter  being  plotted  as  abscissas.  The  efficiencies  shown  by 
this  curve  apply  to  the  machine  when  it  delivers  power  to  non- 
inductive  receiving  circuits  (100  per  cent  power  factor).  When 
the  power  factor  is  less  than  100  per  cent,  the  efficiencies  are  less 
than  the  efficiencies  shown  by  the  curve. 

At  full-load  current  output  of  28.1  amperes  per  phase  and 
100  per  cent  power  factor,  the  efficiency  as  represented  by  the  ordi- 
nate  of  the  curve  is  93  per  cent.  At  half-load  current  output  of 
14.05  amperes  per  phase,  the  efficiency  is  90£  per  cent.  At  full 
load  the  several  losses  in  this  machine  are  as  follows: 

(1)  Friction    and    windage    loss    (obtained    by    experiment), 
2,000  watts. 

(2)  PR  loss  in  the  armature  (calculated  from  hot  resistance 
of  armature  1.46  ohm  per  phase,  and  full-load  armature  current 
28.1  amperes  per  phase)  is 

2  X  (28.1)2  X  1.46  =  2,305  watts 


216  ALTERNATING-CURRENT  MACHINERY 

(3)  PR  loss  in  the  field  (calculated  from  hot  resistance  of  field 
coil  9.35  ohms,  and  full-load  field  current  8.8  amperes)  is 

9.35  X  (8.8)2  -  724  watts 

(4)  Core  loss  (determined  by  experiment  using  the  method 
described  on  page  209),  5,000  watts. 

The  total  loss  is,  therefore 

Friction  and  windage  loss  2,000  watts 

I2R  loss  in  armature  2,305  watts 

PR  loss  in  field  724  watts 

Core  loss  5,000  watts 

Total  loss  10,029  watts 
Therefore 

135  000 
efficiency  at  full-load  -  m,m'+W,«l»  =  93  per  cent 

SYNCHRONOUS  MOTORS 

Motors  designed  to  be  operated  with  alternating  currents  may 
be  divided  into  two  classes:  synchronous  motors  and  induction 
motors.  Both  kinds  are  in  common  use;  and  although  there  are  a 
few  other  motors  which  do  not  come  under  the  above  classification, 
yet  by  far  the  larger  part  of  all  the  motors  run  with  alternating  cur- 
rents belongs  to  one  or  the  other  of  these  classes.  The  induction  motor, 
pages  371-420,  having  properties  which  adapt  it  to  a  much  wider 
field  of  application  than  is  possible  with  the  synchronous  motor, 
is  much  the  more  commonly  used. 

Any  Alternator  a  Synchronous  Motor.  When  a  given  armature 
conductor  of  an  alternating-current  generator  is  under  a  north  pole 
of  the  field  magnet,  the  current  in  the  conductor  is  in  a  direction 
such  that  the  force  which  the  field  exerts  on  the  wire  opposes  the 
motion  of  the  armature.  When  the  given  conductor  has  moved 
sufficiently  to  be  under  a  south  pole  of  the  field  magnet,  the  cur- 
rent will  have  reversed  in  direction,  and  the  force  which  the  field 
exerts  on  the  wire  will  still  oppose  the  motion  of  the  armature.  The 
work  done  in  driving  the  armature  against  these  opposing  electro- 
magnetic forces  is  the  work  that  goes  to  maintain  the  alternating 
current  delivered  by  the  alternator. 


ALTERNATING-CURRENT  MACHINERY  217 

Consider  an  alternator  driven  by  a  small  engine  or  auxiliary 
motor.  Let  an  alternating  current  from  an  outside  source  be  forced 
through  the  revolving  armature  of  the  alternator,  the  field  magnet 
of  which  is  supplied  with  a  direct  current  from  an  exciter.  Then 
the  motion  of  the  revolving  armature  will  be  helped  by  the  alter- 
nating current  if  the  following  conditions  are  satisfied: 

(a)  If  the  speed  of  the  armature  is  such  that  a  given  armature  con- 
ductor moves  from  the  middle  of  a  north  pole  to  the  middle  of  a  south  pole 
during  the  time  of  one  alternation  (half  a  cycle)  of  the  supplied  alternating 
current. 

(b)  If  the  direction  of  the  supplied  alternating  current,  when  the  given 
conductor  is  under  a  north  pole,  is  such  that  the  force  exerted  upon  the  con- 
ductor by  the  field  helpsv  the  motion  of  the  armature. 

This  is  evident  when  we  consider  that  the  current  reverses 
every  time  the  given  conductor  passes  from  one  field  pole  to  the 
next'  and  that  this  reversed  current  will  be^ffcted  upon  by  the  re- 
versed polarity  of  the  next  pole  with  a  force  always  in  the  direction 
of  the  motion. 

When  the  alternator  speed  has  been  carefully  adjusted  so  that 
the  conditions  (a)  and  (b)  are  satisfied,  the  driving  engine  or  auxiliary 
motor  may  be  disconnected;  the  armature  of  the  alternator  will 
continue  to  revolve  at  constant  speed  (the  frequency  of  the  sup- 
plied alternating  current  being  constant),  and  the  revolving  arma- 
ture may  deliver  power  by  belt  to  drive  machinery.  An  alt:rnator 
used  in  this  way  is  called  a  synchronous  motor. 

Any  alternator  may  be  used  as  a  synchronous  motor  without 
alteration  of  any  kind.  Electrically  and  mechanically,  the  syn- 
chronous motor  is  the  same  as  the  alternating-current  generator. 
In  fact  the  same  machine  is  often  used  indifferently  as  motor  or 
generator  according  to  circumstances.  Composite  field  winding  is 
never  provided  on  an  alternator  which  is  to  be  used  as  a  synchronous 
motor. 

Synchronous  motors  may  be  designed  to  operate  either  on 
single-phase  or  polyphase  systems,  and  are  called  synchronous  be- 
cause they  always  run  in  synchronism  with,  i.e.,  at  the  same  fre- 
quency as,  the  alternator  supplying  the  current  to  them.  The 
speed  of  the  motor  cannot  change  unless  the  speed  of  the  generator 
changes;  but  it  is  not  necessarily  the  same  speed  as  that  of  the  gen- 


218  ALTERNATING-CURRENT  MACHINERY 

erator.  The  speed  of  the  motor  will  be  the  same  as  the  speed  of  the 
generator  only  when  the  motor  happens  to  have  the  same  num- 
ber of  poles  as  the  generator.  The  speed  at  which  a  synchronous 
motor  will  run  when  connected  to  an  alternator  supplying  current 
at  a  frequency  /,  is 

2X/X60 

s  —  • 

P 

where  s  is  speed  of  the  motor  in  revolutions  per  minute;  /  is  fre- 
quency of  the  alternating-current  supply;  and  p  is  number  of  poles 
on  the  motor  field.  For  example,  if  a  10-pole  motor  were  run  from 
a  60-cycle  alternator,  the  speed  of  the  motor  would  be 

2  X  60  X  60 

-JQ—        =  720r.-p.-ai 

Moreover,  it  follows  that  if  the  motor  had  the  same  number  of 
poles  as  the  alternator,  the  speed  of  the  motor  would  be  just  the 
same  as  the  speed  of  the  alternator,  and  any  variation  in  the  speed 
of  the  latter  would  cause  a  corresponding  change  in  the  speed  of 
the  motor;  or  if  the  motor  had  half  as  many  poles  as  the  generator, 
its  speed  would  be  double  that  of  the  latter,  and  any  change  in  the 
speed  of  the  generator  would  cause  a  proportional  change  in  that 
of  the  motor;  in  other  words,  the  cyclic  speeds  must  be  the 
same. 

Advantages.  The  synchronous  motor,  especially  in  units  of 
large  output,  possesses  a  number  of  features  which  make  its  use  at 
times  preferable  to  that  of  the  induction  motor.  Its  advantages 
may  be  briefly  summed  up  as  follows : 

(a)  Unvarying  speed  at  all  loads. 

(b)  Power  factor,  variable  at  will  by  change  of  the  exciting  current, 
can  be  made  approximately  unity  at  any  load. 

(c)  The  current  in  the  armature  can  be  made  to  lead  the  electromotive 
force  by  over-exciting  the  field  magnets,  thus  producing  the  same  effect  as  a 
large  condenser.      The  leading  current  in  the  armature  can  be  used  to  neutralize 
the  unfavorable  effects  of  inductance  (which  causes  lagging  currents)  in  other 
parts  of  the  system. 

(d)  The  synchronous  motor  is  cheaper  to  build,  especially  at  low  speeds, 
than  the  induction  motor. 

(e)  Its  efficiency  is  generally  higher  than  that  of  the  induction  motor. 

(f)  It  is  specially  adapted  to  high-voltage  winding. 


ALTERNATING-CURRENT  MACHINERY  219 

Disadvantages.  The  synchronous  motor,  on  the  other  hand, 
has  several  disadvantages,  as  follows: 

(a)  It  is  not  adapted  to  work  requiring  variable  speed,  as  no  independ- 
ent speed  regulation  is  possible. 

(b)  It  has  small  starting  torque;  hence  it  is  not  suitable  for  work  re- 
quiring large  starting  torque,  or  frequent  starting  of  the  load. 

(c)  It  has  a  tendency  to  "hunt." 

(d)  It  requires  an  exciting  current  which  must  be  supplied  by  an  out- 
side source. 

(e)  It  requires  the  most  skilful  and  intelligent  attention. 

Synchronous  motors  are  used  where  power  is  required  in  large 
amounts,  and  where  the  motor  does  not  have  to  be  started  and 
stopped  frequently. 

Comparison  of  Synchronous  and  D.  C.  Motors.  Synchronous 
motors  behave  differently  from  direct-current  motors.  Thus,  if 
the  field  of  a  direct-current  motor  be  weakened,  the  motor  will  speed 
up  in  order  to  keep  the  counter-electromotive  force  at  a  proper 
value.  But  if  the  field  strength  of  a  synchronous  motor  be  changed, 
the  speed  cannot  change,  because  the  motor  must  run  in  synchronism 
with  the  alternator  that  supplies  it  with  current.  What  then  does 
enable  a  synchronous  motor  to  adjust  itself  to  changes  of  load  and 
field  strength?  It  is  the  change  of  phase  difference  between  the  arma- 
ture current  and  the  applied  electromotive  force. 

Suppose  a  synchronous  motor  is  running  light,  i.  e.,  unloaded, 
and  suppose  further  that  there  is  no  friction,  hysteresis,  or  eddy- 
current  losses ;  then,  if  this  motor  be  run  up  to  synchronism,  and  its 
field  adjusted  to  such  a  strength  that  the  counter-electromotive 
force  of  the  motor  is  equal  and  opposite  to  that  of  the  generator, 
no  current  will  flow  through  the  circuit  when  the  circuit  is  closed. 
At  any  instant  the  electromotive  force  that  causes  current  to  flow 
in  the  circuit  is  the  difference  between  the  instantaneous  electro- 
motive force  of  the  alternator  and  the  counter-electromotive  force 
of  the  motor.  On  loading  the  motor,  its  armature  will  lag  a  small 
fraction  of  a  revolution  behind  that  of  the  alternator,  so  that  the 
counter-electromotive  force  of  the  motor  will  no  longer  be  in  op-, 
position  to  that  of  the  alternator.  -The  result  is  that  there  will  be  a 
current  of  a  ^r*\gnitude  such  as  to  produce  the  torque  necessary  to 
enabk  }he  motor  \o  ~arry  its  load.  The  greater  the  load  on  the 
rootor,  Aie  greater  the  lag  of  its  armature,  and  hence  the  greater 


220  ALTERNATING-CURRENT  MACHINERY 

the  difference  in  phase  between  the  applied  and  the  counter-electro- 
motive forces;  this,  in  turn,  permits  a  larger  current  to  flow  to  supply 
the  additional  torque  required.  If,  however,  too  great  a  load  is  put 
on  the  motor,  the  slipping  behind  of  the  armature  will  become 
sufficiently  great  to  throw  the  motor  out  of  synchronism,  or  to 
cause  the  motor  to  "break  down,"  when  it  will  stop.  In  other 
words,  the  motor,  under  these  conditions,  cannot  exert  sufficient 
torque  to  handle  the  load.  1 

Starting  the  Motor.  Single-Phase  Circuit.  If  a  single-phase 
alternator  be  electrically  connected  to  alternating-current  supply 
mains,  the  machine  will  not  start  up  and  run  as  a  motor,  because 
the  current  in  its  armature  is  rapidly  reversing,  thus  tending  to 
turn  the  armature  first  in  one  direction  and  then  in  the  other  direc- 
tion in  rapid  succession.  Such  a  synchronous  motor,  therefore, 
whether  loaded  or  unloaded,  must  always  be  started  and  brought 
up  to  full  speed  by  an  engine  or  other  outside  source  of  power,  since 
it  develops  no  starting  torque  whatever  when  thrown  in  circuit  at 
a  standstill. 

Polyphase  Circuit.  If  a  polyphase  alternator,  on  the  other 
hand,  is  connected  to  polyphase  supply  mains,  the  machine  will 
start  and  run  up  to  full  speed  if  it  has  little  or  no  belt  load.  This 
self-starting  property  of  the  polyphase  synchronous  motor  is  ex- 
plained as  follows: 

As  one  of  the  polyphase  currents  through  the  armature  of  the  machine 
dies  away,  it  leaves  a  slight  amount  of  residual  magnetism  in  the  field-magnet 
structure  if  the  field  magnet  is  not  excited  by  direct  current.  This  residual 
magnetism  acts  upon  the  growing  current  of  the  other  phase  (or  phases),  and 
produces  a  torque  tending  to  turn  the  armature.  This  action  of  the  polyphase 
alternator  is  essentially  the  same  as  the  action  of  the  induction  motor. 

A  polyphase  alternator  which  is  to  be  used  as  a  synchronous 
motor  develops  but  little  starting  effort  when  connected  directly  to 
the  supply  mains,  hence  it  is  generally  started,  especially  in  larger 
sizes,  by  means  of  a  small  engine  or  auxiliary  motor,  and  then  thrown 
into  circuit,  after  which  its  load  is  connected  to  it  through  a  fric- 
tion clutch,  thus  allowing  it  to  be  thrown  on  gradually.  In  smaller 
sizes  the  machine  is  generally  self-starting  without  load,  the  load 
being  thrown  on  afterwards. 

Exciter.  The  exciter  of  a  synchronous  motor  (a  small  direct- 
current  dynamo)  is  usually  belted  to,  or  mounted  upon,  the  synchro- 


ALTERNATING-CURRENT  MACHINERY  221 

nous  motor  shaft,  so  that  when  the  synchronous  motor  has  been 
brought  up  to  full  speed  either  by  separate  starting  or  by  self-starting, 
the  exciter  is  in  full  operation  and  is  in  readiness  to  supply  current 
for  exciting  the  field  magnet  of  the  synchronous  motor. 

Separate  Starting.  In  the  case  of  the  single-phase  machine, 
the  power  for  starting  is  always  derived  from  a  source  entirely  inde- 
pendent of  the  single-phase  supply.  In  the  case  of  the  polyphase 
machine,  the  power  for  starting  is  usually  developed  by  a  small 
induction  motor  supplied  with  polyphase  currents  from  the  mains 
that  supply  currents  to  the  synchronous  motor  itself.  In  other 
words,  the  method  of  separate  starting  is  essentially  the  same  for 
both  single-phase  and  polyphase  machines,  the  procedure  being 
exactly  the  same  as  for  starting  and  adjusting  an  alternator  that  is 
to  be  connected  in  parallel  with  another  alternator  already  in  opera- 
tion. 

Self-Starting  of  the  Polyphase  Type.  In  the  self-starting  of  the 
polyphase  synchronous  motor,  its  armature  terminals  may  be  con- 
nected directly  to  the  polyphase  supply  mains  with  its  field  unexcited. 
When  the  machine  reaches  synchronous  speed  (driving  its  exciter), 
the  field  is  connected  to  the  exciter.  The  machine  is  then  in  full 
operation  as  a  synchronous  motor,  and  its  load  may  be  gradually 
thrown  on. 

The  objection  to  this  mode  of  starting  is  that  the  machine 
takes  excessively  large  lagging  currents  at  starting;  and  this  gen- 
erally causes  a  drop  in  the  supply  voltage  great  enough  to  disturb 
seriously  the  general  system  of  distributing  mains  from  which  the 
synchronous  motor  receives  its  currents.  This  excessive  demand 
for  current  at  starting  is  objectionable  when  the  motor  takes  a 
large  proportion  of  the  generator  output,  or  is  used  in  connection 
with  a  lighting  service,  especially  when  the  motor  is  started  and 
stopped  at  frequent  intervals. 

To  avoid  an  excessive  demand  for  current  at  starting,  an  auto- 
starter  or  compensator  is  frequently  employed.  The  starting  com- 
pensator for  a  two-phase  synchronous  motor  consists  of  two  trans- 
formers (three  transformers  for  a  three-phase  machine)  having  their 
primaries  connected  across  the  respective  phases  of  the  supply 
mains,  their  secondaries  being  provided  with  a  number  of  taps  so 
that,  at  starting,  a  fraction  of  the  full  supply  voltage  can  be  applied 


222  ALTERNATING-CURRENT  MACHINERY 

to  the  armature  terminals  of  the  synchronous  motor.  This  fraction 
is  usually  from  40  to  60  per  cent  of  the  full  voltage;  and  a  switch- 
ing device  is  provided  by  means  of  which  the  change  from  frac- 
tional to  full  voltage  can  be  quickly  made  when  the  synchronous 
motor  reaches  full  speed.  This  starting  compensator  is  also  used  in 
connection  with  induction  motors.  The  transformers  used  in  the 
starting  compensator  are  always  autotransformers. 

The  self-starting  of  a  polyphase  synchronous  motor  (by  induc- 
tion-motor action)  depends  upon  the  magnetizing  action  on  the 
unexcited  field  magnet  poles  by  the  armature  currents.  Therefore, 
a  polyphase  alternator  having  high  armature  reaction,  that  is,  large 
magnetizing  action  on  the  field  for  a  given  armature  current,  as  in 
the  case  of  an  armature  with  concentrated  windings  or  a  machine 
with  small  air  gaps,  will  give  a  large  starting  torque  when  used  as 
a  self -starting  synchronous  motor.  The  revolving  field-structures 
of  self -starting  synchronous  motors  are  usually  provided  with  squirrel- 
cage  windings  to  increase  the  starting  torque  when  used  with  start- 
ing compensators,  and  to  reduce  the  tendency  to  hunt. 

Usually  less  than  one  minute  is  required  to  bring  even  large 
synchronous  motors  to  full  speed  by  either  method  of  starting. 

At  the  time  of  starting,  the  armature  and  field  windings  of  a 
synchronous  motor  are  related  to  each  other  as  are  the  primary  and 
the  secondary  of  an  alternating-current  transformer.  The  result  is, 
that  when  the  field  coils  have  many  turns  of  wire,  a  dangerously 
high  electromotive  force  may  be  induced  in  them. 

This  production  of  high  voltages  in  the  field  coils  of  a  self- 
started  polyphase  synchronous  motor  may  be,  to  a  great  extent 
avoided,  by  using  few  turns  of  large  wire  in  the  field  winding,  thus 
necessitating  the  use  of  a  low-voltage  exciter.  For  this  reason  ex- 
citers giving  an  electromotive  force  as  low  as  50  volts  are  frequently 
used.  Another  method  of  obviating  the  danger  referred  to,  is  to 
provide  short-circuited  metal  rings  around  the  field  poles.  These 
rings  limit  the  changes  of  magnetism  in  the  pole-pieces,  and  thereby 
prevent  the  formation  of  excessively  high  induced  voltages  in  the 
field  coils. 

In  synchronous  motors  of  the  stationary  field  type,  the  field 
circuit  may  be  broken  up  into  many  separate  parts  so  as  to  divide 
up  the  induced  electromotive  force.  Thus,  Fig.  198  shows  the 


ALTERNATING-CURRENT  MACHINERY  223 

stationary  field  of  an  alternator  (synchronous  motor)  with  the  ter- 
minals of  each  field  spool  brought  out  to  convenient  switches  on 
the  frame  of  the  machine.  During  starting  these  switches  are  open, 
and  when  the  machine  has  reached  synchronous  speed,  they  are  all 
closed,  thus  connecting  all  the  field  spools  in  series  to  the  exciter. 

Hunting  Action.  When  the  load  on  a  synchronous  motor  is 
suddenly  increased,  the  motor  slows  down  momentarily  and  falls 
behind  the  generator  in  phase.  When  the  motor  has  fallen  behind 
sufficiently  to  take  in  power  enough  to  enable  it  to  carry  its  load, 
it  is  still  running  slightly  below  synchronism;  it,  therefore,  falls  still 
further  behind,  and  takes  an  excess  of  power  from  the  generator, 


Fig.  198.     Diagrammatic   View   of   Stationary  Field  of  a  Syn- 
chronous Motor 

which  quickly  speeds  it  above  synchronism.  It  then  gains  on  the 
generator  in  phase  until  it  is  taking  in  less  power  than  is  required 
for  its  load,  when  it  again  slows  down,  and  so  on.  This  oscillation 
of  speed  above  and  below  synchronism,  called  hunting,  is  accompanied 
by  great  changes  in  the  current  supplied  to  the  synchronous  motor, 
and  by  rapid  rise  and  fall  of  the  electromotive  force  between  the 
terminals  of  the  motor.  It  is  frequently  a  source  of  great  annoy- 
ance, especially  where  several  synchronous  motors,  or  rotary  con- 
verters, are  run  in  parallel  from  the  same  mains. 

Hunting  is  frequently  produced  by  the  periodic  changes  in  the 
speed  of  the  engine  that  drives  the  generator.  Thus  the  engine 
momentarily  increases  its  speed  as  the  steam  acts  upon  the  piston 


224 


ALTERNATING-CURRENT  MACHINERY 


at  each  stroke,  and  diminishes  its  speed  in  the  intervals  between 
the  strokes. 

The  hunting  of  a  synchronous  motor  is  a  phenomenon  of  the 
same  nature  as  the  hunting  of  a  steam  engine  having  an  over-sen- 
sitive governor.  When  the  load  on  the  engine  is  suddenly  increased, 
the  engine  slows  down  momentarily,  causing  the  governor  to  admit 
more  steam  than  is  needed  for  the  increased  load.  The  result  is 
that  the  engine  quickly  speeds  up,  causing  the  over-sensitive  governor 
to  shut  off  too  much  steam,  so  that  the  engine  slows  down  again, 
and  SQ  on.  Hunting  is  associated  with  more  or  less  violent  shifting 
of  the  resultant  magnetic  flux  in  the  air  gap  under  the  poles.  This 

is  due  to  the  fluctuations 
in  the  armature  current, 
caused  by  the  flow  of  the 
"corrective"  current  be- 
tween generator  and  alter- 
nator while  hunting  occurs. 
Reduction  by  Use  of 
Dampers.  Hunting  is  greatly 
reduced  or  entirely  elimi- 
nated by  the  use  of  heavy 
copper  frames  or  dampers 
in  the  neighborhood  of  the 
poles.  One  form  of  damper 
is  shown  in  Fig.  199,  which 
is  a  view  of  a  portion  of  the  revolving  field  of  an  alternator. 
The  dampers  consist  of  rectangular  copper  frames  driven  into 
place  under  the  overhanging  tips  of  two  adjacent  poles.  A  damper 
is  provided  between  each  pair  of  adjacent  poles,  all  around  the  field, 
both  in  the  alternator  and  in  the  synchronous  motor. 

Another  form  of  damper  which  has  been  found  very  effective 
is  called  the  "squirrel-cage"  damper.  Heavy  bars  of  copper  are 
placed  in  slots  at  the  surface  of  the  poles  and  their  ends  bolted  to  two 
closed  copper  rings,so  as  to  short-circuit  all  the  bars.  This  cage- 
damper  applied  to  a  rotor  for  use  as  a  synchronous  motor  is  illus- 
trated in  Fig.  200,  which  relates  to  a  200-kilowatt  "type  E"  machine 
of  the  Westinghouse  Company. 

The  principle  of  the  damping  action  is  that  the  shifting  magnetic 


Fig.  199. 


Portion  of  Revolving  Field  Showing 
Copper  Frame  Dampers 


ALTERNATING-CURRENT  MACHINERY 


225 


field  sets  up  induced  currents  in  the  short-circuited  frames  or  bars  of 
the  damper,  and  these  currents  react  on  the  magnetic  field  so  as  to 
oppose  the  shifting  of  the  flux  and  thereby  dampen  the  hunting  oscil- 
lations. The  electrical  effect  of  the  dampers  is  analogous  to  the 
mechanical  effect  produced  by  immersing  the  bob  of  a  swinging  pen- 
dulum in  a  heavy  oil  which  resists  the  motion  of  the  bob. 


Fig.  200.     Rotor  of  a  Synchronous  Motor  Provided  with  "Squirrel- 
Cage"   Form  of  Damper 

Torque  and  Power  Output.  A  given  synchronous  motor  operat- 
ing with  given  applied  voltage  is  capable  of  developing  a  definite 
maximum  torque,  or  of  delivering  a  definite  maximum  amount  of 
power.  An  attempt  to  take  more  than  this  maximum 
power  from  the  machine,  causes  the  machine  to  fall  out  of  step, 
that  is,  out  of  proper  phase  relation,  with  respect  to  the  supply 
voltage;  and  the  motor  accordingly  stops.  A  properly  designed 
synchronous  motor  will,  however,  carry  a  reasonable  overload  befort, 
reaching  the  above-mentioned  maximum  at  which  the  machine 
stops. 


226 


ALTERNATING-CURRENT  MACHINERY 


The  maximum  power  that  can  be  delivered  by  a  synchronous 
motor  is  greatly  increased  by  increase  of  the  applied  voltage,  and 
greatly  decreased  by  a  decrease  of  the  applied  voltage.  In  fact  the 
maximum  power. output  is  proportional  to  the  square  of  the  applied 
voltage;  therefore,  the  voltage  of  supply  should  never  be  allowed 
to  fall  much  below  the  normal  or  rated  value. 

Field  Excitation  and  Power  Factor.  While  the  power  factor 
of  a  non-synchronous  (induction)  alternating-current  motor  is  fixed 
by  its  design,  and  its  current  is  always  lagging  behind  the  applied 
electromotive  force,  the  current  delivered  to  a  synchronous  motor 
may  be  made  either  lagging  or  leading  at  will.  This  remarkable 
control  of  the  phase  of  the  current  is  accomplished  by  varying  the 

strength   of  the   field  exci- 
tation. 

An  increase  in  the  field 
excitation  of  a  synchronous 
motor  will  cause  a  corre- 
sponding increase  in  the 
counter-electromotive  force 
generated  in  the  motor  ar- 
mature. By  properly  ad- 
justing the  field  excitation, 
this  counter-electromotive 


Fig.  201.     Curve  Showing  Variation  of  Armature 
Current  with  Field  Excitation 


current, 

force  of  the  motor  can  be 
made  considerably  greater 
than  the  electromotive  force  applied  at  the  motor  terminals.  The 
result  is  that  an  increased  but  leading  current,  that  is,  one  ahead 
of  the  applied  voltage,  as  in  a  condenser,  flows  in  the  armature. 
On  the  other  hand,  a  field  excitation  below  the  normal  amount 
produces  an  increased  but  lagging  current,  that  is,  one  behind  the 
applied  voltage,  as  in  an  inductance  coil.  If  the  field  excitation 
is  normal,  that  is,  of  such  a  value  that  the  current  in  the  motor 
armature  is  exactly  opposed  in  phase  to  the  counter-electromotive 
force  (unity  power  factor),  then  the  effective  value  of  this  current 
will  be  a  minimum;  and  hence  the  efficiency  of  the  motor,  generator, 
and  transmission  lines  will  be  a  maximum  because  the  PR  losses 
will  be  minimum. 

Considered  simply  as  a  motor,  without  reference  to  the  trans- 


ALTERNATING-CURRENT  MACHINERY 


227 


mission  system  as  a  whole,  the  most  efficient  point  of  operation  is 
with  unity  power  factor,  or,  in  other  words,  with  a  field  excitation 
which  will  make  the  armature  current  a  minimum. 

The  effect  upon  the  armature  current,  produced  by  varying  the 
field  excitation,  is  shown  by  the  curves  in  Fig.  201.  Up  to  a  certain 
point,  as  the  excitation  is  increased,  the  armature  current  is  lagging, 
and  decreases  to  a  minimum  value.  Further  increase  of  the  exciting 
current  causes  the  armature  to  take  more  current,  which  is  now  ahead 
of  the  applied  electromotive  force  in  phase,  that  is,  is  now  leading. 
There  is  one  value  of  the  exciting  current  for  which  the  armature 
current  is  a  minimum.  In  motors  of  good  regulation  this  value  of 
the  exciting  current  varies  but  slightly  with  different  loads. 

Use  as  a  Condenser.  A  synchronous  motor  with  its  field  mag- 
net over-excited  takes  a  current  which  is  ahead  of  the  applied  electro- 
motive force  in  phase.  Such  a  machine  may,  therefore,  be  connected 


la 


Fig.  202.      Diagram  Showing  Synchronous  Motor  Used  as  a  Condenser 

across  the  terminals  of  an  inductive  receiving  circuit,  as  shown  in 
Fig.  202,  so  as  to  compensate  for  lagging  current  delivered  to  the 
receiving  circuit,  thus  reducing  the  line  current  to  the  lowest  value 
that  will  suffice  to  transmit  the  power  taken  by  the  receiving  circuit. 
The  clock  diagram,  Fig.  203,  shows  how  the  leading  current  72  taken 
by  the  synchronous  motor  M,  Fig.  202,  gives,  when  combined  with 
the  current  I\  delivered  to  the  receiving  circuit,  a  resultant  line 
current  /,  which  is  in  phase  with  the  electromotive  force  E  between 
the  mains. 

A  synchronous  motor  used  primarily  for  compensating  the 
lagging  current  delivered  to  an  inductive  receiving  circuit  is  called 
a  rotary  condenser  or  a  synchronous  compensator.  The  rotary  con- 
denser is  especially  useful  when  induction  motors  or  lightly  loaded 


228  ALTERNATING-CURRENT  MACHINERY 

transformers  or  both,  are  supplied  over  a  long  transmission  line. 
In  such  cases  the  reduction  of  the  line  current  to  the  smallest  possible 
value  effects  considerable  saving  in  the  matter  of  power  losses  in 
the  transmission  line.  The  use  of  a  rotary  condenser  is  also  an  ad- 
vantage in  that  the  regulation  of  voltage  at  the  receiving  end  of  the 
transmission  line  is  improved. 

A  given  synchronous  motor  is  most  effective  as  a  rotary  con- 
denser when  it  is  not  required  to  deliver  any  mechanical  power  as  a 
motor,  that  is,  when  it  is  run  at  zero  load.  When  a  synchronous 
motor  is  to  be  used  to  deliver  mechanical  power,  as  well  as  to  take  a 
leading  current  for  the  purpose  of  compensating  the  lagging  current 
taken  by  the  inductive  receiving  circuit,  it  is  customary  to  limit 
the  load  on  the  motor  to  70.7  per  cent  of  its  full-load  rating,  that  is, 
its  rating  if  it  were  to  be  used  as  a  motor  only,  and  not  as  a  rotary 
condenser.  When  the  motor  takes  in  its  full-load  rated  current, 
but  only  70.7  per  cent  of  its  full-load  rated  power,  its  field  being 

over-excited,    then    its  current  is  45 
degrees     ahead    of    the    e.     m.    f.; 
the   power   factor   of    the   motor   is 
E          70.7  per  cent;  the  power  component 
of  the  current,  that  is,   the   compo- 
nent which  is  in  phase  with  the  sup- 
ply electromotive  force,    is  70.7  per 
,-,.    ono     ni  cent    of    the    full    current:  and  the 

Fig.  203.      Clock   Diagram   of    Cur- 
rent Relations^ Synchronous         wattless    component    of  the    current 

(the  component  which  is  90  degrees 

ahead  of  the  supply  electromotive  force  in  phase)  is  also  70.7  per 
cent  of  the  full-load  current. 

For  example,  a  synchronous  motor  rated  at  100  kilowatts  at 
1,000  volts  would  have  a  full-load  rated  current  of  100  amperes, 
the  field  of  the  machine  being  excited  to  give  unity  power  factor. 
If  this  machine  is  to  be  operated  as  a  rotary  condenser,  its  field 
excitation  will  have  to  be  increased.  If  the  machine  is  to  carry  a 
load  of  70.7  kilowatts,  and  at  the  same  time  to  take  as  large  a  lead- 
ing wattless  current  as  possible  (without,  however,  exceeding  the 
full-load  rated  current  of  100  amperes),  then  the  field  will  have  to  be 
over-excited  to  such  an  extent  as  to  cause  the  machine  to  take  its 
full  rated  current  of  100  amperes  at  70.7  kilowatts  load.  In  this 


ALTERNATING-CURRENT  MACHINERY  229 

case  the  power  component  of  the  100-ampere  current  will  be  70.7 
amperes;  the  wattless  component  of  the  100-ampere  current  will 
be  70.7  amperes  [lOO  =  1/(70.7)2+  (70.7)2];  and  the  power  factor 
of  the  over-excited  motor  will  be  70.7  per  cent. 

MOTOR  TESTING 

The  difference  between  a  synchronous  motor  and  an  alternating- 
current  generator  consists  mainly  in  the  method  of  operating;  any 
alternator  will  run  as  a  synchronous  motor,  and  vice  versa.  There- 
fore, all  the  tests  described  on  pages  188-216,  with  reference  to  alter- 
nators, may  be  applied  in  a  similar  manner  to  synchronous  motors. 
In  making  a  heat  test  on  a  small  synchronous  motor,  it  is 
usually  run  at  full  load  as  a  motor. 

In  the  case  of  large  synchronous  motors,  the  "heat-run"  or 
test  is  usually  made  by  running  the  machine  as  a  generator  on  short- 
circuit,  with  a  portion  of  its  field  coils  connected  in  opposition,  as 
described  on  page  205  on  the  heat  test  of  alternators. 

The  efficiency  of  a  synchronous  motor  is  calculated  in  the  same 
manner  as  that  of  an  alternator,  page  214. 

Phase  Characteristic.  Regulation  for  alternators,  as  described 
on  page  200,  is  not  calculated  in  the  case  of  a  synchronous  motor; 
but  the  determination  of  the  "phase  characteristic"  of  a  synchro- 
nous motor  corresponds  to,  and  is  substituted  for,  the  regulation 
test.  A  "phase  characteristic"  is  a  curve  showing  the  relation  be- 
tween the  armature  current  and  the  field  current  of  a  synchronous 
motor,  the  test  being  carried  out  under  constant  conditions  with 
respect  to  voltage,  frequency,  and  load.  Phase  characteristics  are 
shown  in  Fig.  201. 

Phase  characteristics  are  usually  taken  at  no  load,  although  it 
not  infrequently  happens  that  it  is  desired  to  obtain  a  phase  char- 
acteristic at  full  load.  To  make  the  test  at  no  load,  it  is  simply 
necessary  to  run  the  motor  unloaded,  supplying  alternating  currents 
of  the  proper  voltage  and  frequency  to  the  armature  terminals 
of  the  motor.  The  field  current  is  then  varied  by  successive  steps,, 
both  above  and  below  its  normal  value,  until  the  armature  current 
has  attained  rated  full-load  value,  both  for  leading  and  for  lagging 
current.  For  each  value  of  the  field  current,  simultaneous  obser- 
vations are  made  of  the  amperes  in  the  armature,  the  volts  supplied 


230  ALTERNATING-CURRENT  MACHINERY 

to  the  armature  terminals,  the  amperes  in  the  field,  and  the  speed. 
The  normal  field  current  is  that  value  for  which  the  armature  cur- 
rent has  the  lowest  value. 

If  a  phase  characteristic  at  full  load  is  desired,  the  most  con- 
venient wray  of  loading  the  motor  is  to  cause  it  to  drive  a  direct- 
current  generator  of  known  efficiency,  by  means  of  a  belt  connecting 
their  respective  pulleys.  The  power  output  of  the  direct-current 
generator  can  be  accurately  measured  by  means  of  an  ammeter 
and  a  voltmeter.  Knowing  the  output,  and  the  efficiency  of  the 
direct-current  generator  at  any  output,  the  mechanical  input,  to  the 

generator,  that  is, -^r-r .can  be  calculated.     This  input  to  the 

efficiency 

generator  is  evidently  equal  to  the  mechanical  output  of  the  syn- 
chronous motor,  the  power  lost  in  the  belting  being  negligible. 

The  load  on  the  motor  must  be  kept  constant  throughout  the 
test.  The  armature  current  for  a  full-load  phase  characteristic  cannot 
be  varied  through  such  a  wide  range  of  values  as  at  no  load,  owing 
to  the  inability  of  the  armature  windings  of  the  motor  to  carry  the 
excessive  current  without  over-heating. 

Pulsation  Test.  This  test  is  to  determine  whether  or  not  the 
synchronous  motor  has  a  decided  tendency  to  hunt.  For  this  test 
the  synchronous  motor  is  supplied  with  alternating  current  or 
currents  from  a  very  steadily  driven  alternator  over  an  "artificial 
transmission  line."  This  "artificial  line"  consists  simply  of  a  resist- 
ance equal  to  the  resistance*  of  the  transmission  line  over  which 
the  synchronous  motor  is  to  be  supplied  with  current  when  finally 
installed. 

The  synchronous  motor  is  driven  at  zero  load,  taking  current 
through  the  artificial  line,  the  tendency  to  hunt  being  greatest  at 
zero  load.  The  connections  are  the  same  as  in  the  test  for  phase 
characteristic.  The  hunting  action  is  indicated  by  the  swinging  to 
and  fro  of  the  pointers  of  the  ammeters  and  voltmeters.  The  field 
current  of  the  motor  is  varied  from  considerably  below  to  consider- 
ably above  rated  full-load  field  current.  For  each  value  of  the  field 
current,  the  indications  of  the  instruments  are  carefully  observed; 
the  pulsations  of  the  instruments  are  noted. 


*Usually  the  transmission   line  has  a  resistance  such  as  to  give  a  10  per  cent  drop  of 
electromotive  force  when  full-load  current  is  delivered  to  the  motor. 


ALTERNATING-CURRENT  MACHINERY  231 

NOTE.  During  this  test,  care  must  be  taken  that  the  alternator  supply- 
ing the  power  to  drive  the  motor  under  test,  does  not  pulsate,  for,  if  it  does, 
the  pulsations  of  the  instrument  pointers  would  not  then  give  a  reliable  indi- 
cation of  the  performance  of  the  motor -itself  under  normal  working  conditions. 

Break=Down  Test.  As  its  name  implies,  this  test  is  to  deter- 
mine the  maximum  power  that  a  synchronous  motor  will  deliver 
at  its  pulley,  before  falling  out  of  synchronism  and  stopping.  As  in 
the  case  of  the  full-load  phase  characteristic,  the  power  output  of 
the  motor  is  most  conveniently  absorbed  and  measured  by  belting 
the  motor  to  a  direct-current  generator,  and  measuring  the  electrical 
output  of  this  generator.  For  further  description  and  details  of  the 
break-down  test,  the  reader  is  referred  to  the  article  on  the  break- 
down test  for  induction  motors. 

Self=Starting  Test.  The  object  of  a  starting  test  on  a  syn- 
chronous motor  is  to  determine: 

(a)  The  voltage  and  current  required  to  start  the  motor. 

(b)  The  time  required  for  the  motor  to  reach  synchronous  speed  (syn- 
chronism). 

(c)  The  electromotive  force  induced  in  the  field-magnet  windings  at  the 
instant  of  starting. 

The  starting  test  is  made  on  polyphase  motors  only,  for,  as 
previously  stated,  single-phase  synchronous  motors  are  not  inher- 
ently self -starting,  but  must,  even  in  the  smaller  sizes,  be  provided 
with  special  starting  devices. 

The  synchronous  motor  to  be  tested  is  connected  to  mains 
supplying  alternating  currents  of  the  proper  frequency.  Arrange- 
ment is  made  to  adjust  the  voltage  applied  to  the  armature  ter- 
minals of  the  motor  by  means  of  potential  (voltage)  regulators  con- 
nected in  the  armature  circuits  between  the  supply  mains  and  the 
armature  terminals.  An  ammeter  is  connected  in  series  with  each 
phase  winding  of  the  armature,  that  is,  two  ammeters  are  needed 
for  a  two-phase  motor,  and  three  ammeters  for  a  three-phase  motor. 
A  voltmeter  is  connected  to  the  terminals  of  the  secondary  coil  of 
a  small  step-down  potential  transformer,  the  primary  coil  of  which 
is  connected  to  the  terminals  of  the  field  winding  of  the  motor. 
A  voltmeter  is  also  connected  between  the  supply  mains. 

The  test  is  made  with  no  current  in  the  field  and  the  field  cir- 
cuit open.  *  The  voltage  applied  to  the  armature  terminals  is  slowly 

*The  primary  of  the  potential  transformer  takes  but  little  current,  and  the  field  cir- 
cuit, to  all  intents  and  purposes,  is  open. 


232  ALTERNATING-CURRENT  MACHINERY 

increased  until  the  motor  starts  to  revolve.  At  the  instant  that 
the  motor  starts,  the  readings  of  all  the  instruments  are  recorded 
as  follows:  volts  between  armature  terminals;  amperes  in  the  arma- 
ture (each  phase);  and  volts  induced  in  field  windings. 

After  the  motor  starts,  the  voltage  applied  to  the  armature 
terminals  is  kept  constant;  and  the  time  required  for  the  motor  to 
attain  synchronous  speed,  reckoned  from  the  instant  it  starts,  is 
observed.  The  exact  instant  that  synchronism  is  reached,  is  indi- 
cated by  violent  swings  of  the  pointers  of  the  ammeters  as  well  as 
those  of  the  voltmeters  connected  to  the  armature  terminals.  At 
synchronism,  another  set  of  observations  is  taken. 

In  order  to  find  the  most  unfavorable  position  of  the  armature 
and  the  corresponding  starting  current,  and  the  time  required  to 
reach  synchronous  speed  from  this  most  unfavorable  position,  the 
above  procedure  is  repeated  for  a  series  of  initial  positions  of  the 
armature,  chosen  as  follows: 

The  circumference  of  the  armature  between  the  centers  of  two 
adjacent  field  poles  is  divided  into  a  number  of  equal  parts,  this 
number  not  being  a  multiple  of  the  number  of  phases,  nor  of  the 
number  of  slots  per  pole  per  phase;  usually  there  are  seven  parts. 
The  starting  positions  thus  chosen  will  include  every  possible  posi- 
tion that  a  magnet  pole  may  have  relative  to  an  armature  slot. 
These  parts  are  marked  by  chalked  lines,  and  the  various  starting 
positions  of  the  armature  relative  to  the  field-magnet  poles  are  deter- 
mined by  setting  each  of  the  marks  in  succession  into  coincidence 
with  a  given  field  pole  tip, 


ALTERNATING-CURRENT 
MACHINERY 

PART  IV 


TRANSFORMER 

Description.  The  transformer  consists  of  two  separate  and 
distinct  coils  of  wire  insulated  from  each  other,  and  wound  upon  one 
and  the  same  laminated  iron  core.  Fig.  242,  page  273,  shows  a  sec- 
tional view  of  a  commercial  type  of  transformer.  In  practice,  one 
of  the  coils  receives  alternating  current  from  a  high  (or  low)  voltage 
source  of  supply;  and  the  other  coil  delivers  alternating  current  to 
a  receiving  system  at  a  low  (or  high)  voltage.  When  the  transformer 
receives  alternating  current  at  high  voltage  and  delivers  it  at  low 
voltage,  we  have  what  is  called  step-down  transformation;  when  the 
transformer  receives  alternating  current  at  low  voltage  and  delivers 
it  at  high  voltage,  we  have  what  is  called  step-up  transformation. 

The  coil  of  a  transformer  which  receives  alternating  current 
from  a  source  of  supply,  is  called  the  primary  coil;  and  the  coil 
which  delivers  alternating  current  is  called  the  secondary  coil.  In 
Fig.  242,  each  limb  of  the  core  is  wound  with  half  of  the  secondary 
coil  (coarse  wire)  next  to  the  core,  and  with  half  of  the  primary  coil 
(fine  wire)  over  the  secondary. 

The  alternator,  as  we  have  already  seen,  is  a  machine  in  which  an 
alternating  electromotive  force  is  produced  by  the  cutting  of  a 
permanently  established  magnetic  flux  by  wires  on  account  of  the 
motion  of  the  flux  relative  to  the  wires. 

The  transformer,  on  the  other  hand,  is  an  arrangement  whereby 
an  alternating  electromotive  force  is  produced  in  a  stationary  coil 
of  wire  (secondary)  by  reversals  of  magnetic  flux  through  a  station- 
ary iron  core,  these  reversals  of  flux  being  produced  by  alternating 
current  supplied  to  the  primary  coil  of  the  transformer. 

Copyright,  191%,  by  American  School  of  Correspondence. 


234  ALTERNATING-CURRENT  MACHINERY 

Physical  Action.  Without  Load.  When  the  secondary  of  a 
transformer  is  on  open  circuit,  it  can  of  course,  deliver  no  current; 
and  the  transformer  is  said  to  be  operating  at  zero  load.  Under  these 
conditions,  only  a  small  amount  of  alternating  current  flows  through 
the  primary  coil.  This  current  causes  repeated  reversals  of  magnetic 
flux  through  the  iron  core.  These  reversals  of  magnetic  flux  induce 
electromotive  forces  in  both  coils.  The  electromotive  force  thus 
induced  in  the  primary  coil  is  opposite  in  direction,  and  very  nearly 
equal,  to  the  electromotive  force  applied  to  the  primary  coil.  Only 
the  difference  between  the  applied  electromotive  force  and  the 
opposing  induced  electromotive  force  is  available  for  producing 
current  through  the  primary  coil;  and  since  this  difference  is  small, 
the  primary  current  is  small  at  zero  load.  The  primary  current  at 
zero  load  is  called  the  no-load  current  of  the  transformer. 

With  Load.  Whether  the  secondary  coil  of  a  transformer  is 
delivering  current  or  not,  the  reversals  of  magnetic  flux  in  the  trans- 
former core  always  induce  an  alternating  electromotive  force  in  the 
coil.  When  alternating  current  is  taken  from  the  secondary  of  a 
transformer,  the  transformer  is  said  to  be  loaded.  The  action  of  this 
secondary  current  as  it  flows  through  the  secondary  coil,  is  to  oppose 
the  magnetizing  action  of  the  slight  current  already  flowing  in  the 
primary  coil,  thus  decreasing  the  maximum  value  reached  by  the 
alternating  magnetic  flux  in  the  core,  and  thereby  decreasing  the 
induced  electromotive  forces  in  both  coils.  The  amount  of  this 
decrease,  however,  is' very  small,  inasmuch  as  a  very  small  decrease 
of  the  induced  electromotive  force  in  the  primary  coil  greatly  in- 
creases the  difference  between  electromotive  force  applied  to  the 
primary  coil  and  the  opposing  electromotive  force  induced  in  the 
primary  coil,  so  that  the  primary  current  is  greatly  increased.  In 
fact,  the  increase  of  primary  current  due  to  the  loading  of  the  trans- 
former is  just  great  enough  (or  wry  nearly)  to  exactly  balance  the  mag- 
netizing action  of  the  current  in  the  secondary  coil;  that  is,  the  flux  in 
the  core  must  be  maintained  approximately  constant  by  the  primary 
current  whatever  value  the  secondary  current  may  have. 

Electromotive  Force  Relations.  The  electromotive  forces  in- 
duced in  the  respective  coils  of  a  transformer  are  proportional  to 
the  number  of  turns  of  wire  in  each;  and  from  the  above  discussion, 
it  is  evident  that  the  electromotive  force  induced  in  the  primary  coil 


ALTERNATING-CURRENT  MACHINERY  235 

is  sensibly  equal  to  the  supply  electromotive  force,  whether  the 
transformer  is  loaded  or  not.  Therefore 

^--Z/  (32) 

E"     Z" 

in  which  Ef  is  the  electromotive  force  applied  to  the  primary,  E" 
is  the  electromotive  at  which  the  secondary  coil  delivers  alternating 
current,  Zf  is  the  number  of  turns  of  wire  in  the  primary  coil,  -and 
Z"  is  the  number  of  turns  of  wire  in  the  secondary  coil. 

Current  Relations.  The  magnetizing  action  of  the  primary 
current  /'  of  a  transformer  having  Z'  turns  of  wire  in  its  primary 
coil,  may  be  expressed  by  the  product  Z'If,  that  is,  by  ampere-turns; 
and  similarly,  the  magnetizing  action  of  the  secondary  current  /" 
may  be  expressed  by  the  product  Z"ln ',  where  Z"  is  the  number  of 
turns  of  wire  in  the  secondary  coil.  Therefore,  since  the  magnetiz- 
ing actions  of  the  two  coils  are  equal  (and  opposite),  we  have 

ZfI'=Z"l" 
or 

r/          v/r 

T-T  (33) 

in  which  Zf  and  Z"  are  the  turns  of  wire  in  the  respective  coils;  I"  is 
the  current  delivered  by  the  secondary  coil;  and  I'  is  the  increase  of 
current  taken  by  the  primary  coil  over  and  above  the  no-load  cur- 
rent, due  to  the  fact  that  the  secondary  coil  is  delivering  current. 

Now  in  most  commercial  transformers  the  no-load  current  is  quite 
small;  and,  neglecting  this  current  entirely,  the  only  current  in  the 
primary  coil  would  be  the  increase  of  primary  current  due  to  the  fact 
that  the  secondary  coil  is  delivering  current.  Therefore,  equation 
(33)  expresses,  with  sufficient  accuracy  for  most  purposes,  the  relation 
between  the  actual  primary  current  /'  and  the  secondary  current  I". 

Summary  of  Electromotive  Force  and  Current  Relations.  A  trans- 
former which  delivers  current  I"  to  a  receiving  circuit,  takes  an  amount 

Z"  "' 

of  current  equal  to  I"  y(  —  (=  I')  from  the  source  of  supply.     The 

A 

electromotive  force  of  the  source  of  supply  is  Ef,  and  the  electro- 
motive force  at  which  the  secondary  delivers  current  is  equal  to 

E'X~  (=£")• 


236  ALTERNATING-CURRENT  MACHINERY 

Example.  A  certain  transformer  rated  at  5j  kilowatts  has  660  turns  of 
wire  in  its  primary  coil  and  66  turns  of  wire  in  its  secondary  coil.  The  primary 
coil  is  connected  between  1,100-volt  supply  mains.  Therefore,  the  secondary 
electromotive  force,  by  equation  (32)  is 

f7  f  f  A  A 

E"  =  -  -    X  E'  =  -  -  X  1,100  =  110  volts 
Z  660 

If  ten  lamps,  each  taking  half  an  ampere,  are  connected  to  the  secondary 
coil,  the  secondary  current  will  be  5  amperes;  and  the  primary  current,  by 
equation  (33)  will  be 

r*frX7"-J|   X  5  =  .5  ampere 

The  power  delivered  to  the  lamps  by  the  secondary  coil  is  equal  to  E"  I" 
since  the  lamp  circuit  is  non-inductive.  That  is: 

Power  delivered  to  lamps  =  110  volts  X  5  amperes  =  550  watts. 

The  power  delivered  to  the  primary  coil  in  this  case  (non-inductive  sec- 
ondary circuit),  is  equal  to  E'  I'.  That  is: 

Power  delivered  to  primary  =  1,100  volts  X  0.5  amperes  =  550  watts. 

If  100  lamps,  each  taking  half  an  ampere,  are  connected  to  the  secondary 
coil,  the  secondary  current  will  be  50  amperes,  and  the  primary  current  will  be 
5  amperes;  the  power  delivered  to  the  lamps  will  be  5,500  watts,  and  the  power 
delivered  to  the  primary  coil  will  also  be  5,500  watts. 

The  above  calculations  ignore  the  following  actions  which  take 
place  in  an  actual  transformer:  (a)  losses  of  electromotive  force 
in  overcoming  the  resistances  of  primary  and  secondary  coils;  (b) 
losses  of  power  (PR)  in  the  primary  and  secondary  coil;  and  (c) 
loss  of  power  in  the  iron  core,  due  to  hysteresis  and  eddy  currents. 

Automatic  Action  of  the  Transformer.  When  the  load  on  a 
transformer  is  increased,  the  primary  of  the  transformer  automatically 
takes  additional  current  and  power  from  the  supply  mains  in  direct 
proportion  to  the  load  on  the  secondary.  When  the  load  on  the 
secondary  is  reduced,  for  example,  by  turning  off  lamps,  the  power 
taken  from  the  supply  mains  by  the  primary  coil  is  automatically 
reduced  in  proportion  to  the  decrease  in  the  load.  This  automatic 
action  of  the  transformer  due  to  the  balanced  magnetizing  action 
of  the  primary  and  secondary  currents  is  illustrated  in  the  above 
example. 

Ideal  and  Practical  Transformer.  The  foregoing  discussion  of 
electromotive  force  and  current  relations  in  a  transformer  is  based 
upon  the  following  assumptions: 

(a)  That  the  no-load  current  of  the  transformer  is  negligible,  and 
that  it  represents  no  power  taken  from  the  supply  mains;  or,  in  other  words, 
that  eddy-current  and  hysteresis  losses  are  absent. 


ALTERNATING-CURRENT  MACHINERY  237 

(b)  That  the  resistance  of  the  coils  is  negligible,  so  that  the  electro- 
motive force  applied  to  the  primary  coil  is  wholly  balanced  by  the  opposing 
electromotive  force  in  the  primary  coil,  and  so  that  the  whole  of  the  electro- 
motive force  induced  in  the  secondary  coil  is  available  at  the  terminals  of  that 
coil. 

(c)  That  all  the  magnetic  flux  which  passes  through  the  primary  coil 
passes  through  the  secondary  coil  also;  or,  in  other  words,  that  there  is  no 
magnetic  leakage. 

A  transformer  that  would  meet  these  conditions  would  be~arr 
ideal  transformer.  A  well-designed  transformer  operating  on  mod- 
erate load  does  approximate  quite  closely  to  the  ideal  transformer 
in  its  action;  and  equations  (32)  and  (33)  are  much  used  in  practical 
calculations.  For  some  purposes,  however,  it  is  desirable  to  con- 
sider the  action  of  the  transformer,  taking  account  of  coil  resist- 
ances, of  eddy  currents  and  hysteresis,  and  of  the  fact  that  some 
lines  of  magnetic  flux  pass  through  one  coil  without  passing  through 
the  other  (magnetic  leakage).  The  extent  to  which  a  well-designed 
transformer  deviates  from  an  ideal,  is  exemplified  by  the  following 
actual  results  obtained  with  the  5J-kilowatt  transformer  used  in 
the  example  on  the  preceding  page. 

At  no  load,  the  value  of  E"  is  109.8  volts;  the  no-load  current  is  0.129 
amperes;  and  the  power  taken  from  the  mains  by  this  no-load  current  (core 
loss)  is  100  watts.  This  core  loss  is  nearly  constant  at  all  loads. 

When  100  lamps,  taking  50  amperes  of  current,  are  connected  to  the 
secondary,  then  E"  is  107.2  volts.  The  PR  loss  in  the  primary  coil  is  65  watts; 
and  the  PR  loss  in  the  secondary  is  65  watts.  Therefore,  the  power  delivered 
to  the  lamps  is  5.36  Tdlo watts;  the  power  taken  from  the  supply  mains  is 
5,360  +  100  +  65  +  65,  which  is  equal  to  5,590  watts;  and  the  full-load 
efficiency  of  the  transformer  is  96  per  cent. 

Maximum  Core  Flux.  In  the  designing  of  transformers  and  in 
the  predetermination  of  core  loss,  it  is  necessary  to  calculate  the 
maximum  value  reached  by  the  alternating  magnetic  flux  through 
the  transformer  core.  This  maximum  value  of  the  core  flux  may  be 
easily  and  accurately  calculated  in  the  following  manner: 

Let  <£  be  the  maximum  value  of  the  core  flux  (equal  to  the  pro- 
duct EA  of  maximum  flux  density  and  sectional  area  of  the  core) 
E  the  effective  value  of  the  electromotive  force  applied  to  the  primary 
coil,  Z  the  number  of  turns  of  wire  in  the  primary  coil,  and  /  the 
frequency  of  the  applied  electromotive  force,  in  cycles  per  second. 
Now  consider  the  instant  when  the  core  flux  is  at  its  maximum  posi- 


238  ALTERNATING-CURRENT  MACHINERY 

live  value  <I>.    After  a  quarter  of  a  cycle,  or  after—  second,  the  flux 

is  reduced  to  zero.    The  average  rate  of  change  of  the  flux  during 
this  quarter  of  a  cycle  is  equal  to 

total  change  of  flux  _  <&          . 
elapsed  time  1 


which  is  equal  to  the  average  electromotive  force  (in  c.  g.  s.  units) 
induced  in  each  turn  of  wire  in  the  primary  coil.  Therefore,  the 
total  electromotive  force  induced  in  the  Z  turns  of  wire  in  the  primary 
coil  is  4/<f>Z  c.  g.  s.  units,  or  4/<f>Z-:-108  volts,  since  108  c.  g.  s.  units 
equal  1  volt. 

The  average  value  of  the  induced  electromotive  force  is  equal 
(very  nearly)  to  the  average  of  the  electromotive  force  applied  to 
the  primary  coil;  and  it  must  be  multiplied  by  the  form  factor*  of 
the  electromotive  force  curve  to  give  the  effective  value  E  of  the 
applied  electromotive  force.  The  form  factor  of  a  sine-wave  electro- 
motive force  is  1.11.  Therefore 

E=  1.11  X  average  value  =  4.44/$Z-^  108  volts 
Hence,  solving  for  4>,  we  obtain  the  equation 

#V  108 


Example.  In  the  5i-kilowatt  transformer  used  as  an  illustration  on 
page  236,  there  are  660  turns  of  wire  in  the  primary  coil,  that  is  Z  =  660. 
This  primary  coil  is  connected  to  alternating-current  supply  mains  so  that  the 
electromotive  force  applied  to  the  primary  coil  is  1,100  volts  (effective).  The 
frequency  of  the  electromotive  force  is  125  cycles  per  second  (  =/)  .  On  the 
assumption  that  the  electromotive  force  wave  is  a  sine  curve,  we  have,  using 
equation  (34), 

1  100  X  108 


4.44X660X125 

This  is  the  maximum  value  of  the  alternating  magnetic  flux  in  the  transformer 
core. 

The  cross-sectional  area  A  of  the  transformer  core  is  15j  square  inches, 
so  that  the  maximum  value  of  the  magnetic  flux-density  in  the  core  is 

$        300,000 

—  =  —  —  —  —  =  19,350  lines  per  square  inch 
A  15$ 

This  is  equivalent  to  a  flux-density  of  3,000  lines  per  square  centimeter. 
*See  page  15. 


ALTERNATING-CURRENT  MACHINERY 


239 


Ideal  Transformer  Action  Graphically  Represented.  CASE  (a) 
Without  Load.  The  line  0$  in  the  clock  diagram,  Fig.  204,  repre- 
sents the  alternating  magnetic  flux  in  the  core 
of  a  transformer,  the  line  OE'  represents  the 
electromotive  force  applied  to  the  primary  coil, 
and  the  line  OE"  represents  the  electromotive 
force  induced  in  the  secondary  coil.  When 
the  transformer  is  at  zero  load,  the  current  in 
the  primary  coil  (the  no-load  current)  lags  greatly 
behind  the  applied  electromotive  force  E',  as 
shown  in  the  figure,  in  which  the  line  O/o 
represents  the  no-load  current.  The  electro- 
motive forces  induced  in  both  primary  and  Fig.  204.  clock  Diagram 

,  .,  f\r\     1  i     i  •      i    j.1  for  Transformer  with- 

secondary  coils  are  90  degrees  behind  the  core  out  Load 

flux  0<£  in  phase,  and  the  electromotive  force  OE' 
applied  to  the  primary  coil,  being  at  each  instant  opposite  to  the 
electromotive  force  induced  in  the  primary,  is  90  degrees  ahead  of 
0$  in  phase. 

CASE  (b)  With  Load,  Receiving  Circuit  Nearly  Non-inductive. 
The  lines  0<i>,  OE'  OE",  and  OIQ  in  Fig.  205  represent  alternating 
core  flux,  primary  applied  electromotive  force,  secondary  induced 
electromotive  force,  and  no-load  current,  exactly  as  in  Fig.  204.  The 
line  01"  represents  the  secondary  current 
lagging  slightly  behind  the  secondary  electro- 
motive force  OE"\  and  the  line  OA  represents 
the  increase  of  primary  current  due  to  the 
loading  of  the  transformer.  The  total  pri- 
mary current  is  represented  by  the  line  01', 
which  is  the  vector  (or  geometric)  sum  of 
OA  and  070.  The  current  OA  is  exactly 
opposite  to  01"  in  phase;  and  the  product 
of  this  current  OA  and  the  primary  turns  Z' 
balances  the  magnetizing  action  Z"I"  of  the 
secondary  current.  As  is  evident  from  Fig. 
205,  the  loading  of  a  transformer  (non- 
inductive  load)  not  only  increases  the  value 
of  the  primary  current,  but  reduces  its  angle 
of  lag  behind  the  primary  applied  electromotive  force.  Thus,  at  zero- 


rig.  205.     Clock  Diagram 

for  Transformer  with 

Load 


240 


ALTERNATING-CURRENT  MACHINERY 


Fig.  206.     Clock  Diagram  for  Trans- 
former J  with  Load — highly  Induc- 
tive Receiving  Circuit^ 


load  the  primary  current  is  0/0;  and  when  the  transformer  is  loaded 
(non-inductive  load)  the  primary  current  becomes  OF. 

CASE  (c)  With  Load,  Receiving 
Circuit  Highly  Inductive.  In  Fig. 
206  the  line  01"  represents  the  cur- 
rent delivered  by  the  secondary  coil 
of  a  transformer  to  a  highly  induc- 
tive receiving  circuit;  the  line  OA 
represents  the  increase  of  primary 
current  due  to  the  load;  and  OP 
represents  the  total  primary  current. 
In  this  case,  also,  the  part  OA  of 
the  primary  current  is  exactly  op- 
posite in  phase  to  the  secondary  cur- 
rent or. 

Influence  of  Coil  Resistances  and  Magnetic  Leakage.  The 
foregoing  discussion  takes  account  of  the  no-load  current  of  a  trans- 
former. This  no-load  current  is  the  only  factor  that  affects  the  ideal 
relation,  equation  (33),  between  primary  and  secondary  currents  in 
a  transformer.  Coil  resistances  and  magnetic  leakage  are,  on  the 
other  hand,  the  only  things  that  affect  perceptibly  the  ideal  relations, 

equation  (32),  of  primary  and  secondary 
electromotive  forces. 

Magnetic  leakage  is  equivalent,  in  its 
effect  upon  the  action  of  a  transformer,  to 
an  outside  inductance  (a  choke  coil)  con- 
nected in  series  with  the  primary  coil. 
Let  L'  be  this  inductance  (in  henrys) 
which  is  equivalent  to  the  magnetic  leak- 
age of  a  transformer.  Then  wLf  (co  equals 
2n  times  the  frequency)  is  the  reactance 
(in  ohms)  of  this  inductance. 

The  effects  of  coil  resistances  and  mag- 
netic leakage  upon  the  ideal  relation  be- 
tween E'  and  E"  are  shown  in  the  clock 

Fig.  207.     Clock  Diagram  Show- 
ing Effects  of  Coil  Resistance  diagram,  Fig.  207. 
and  Magnetic  Leakage 

The  total  electromotive  force  OEf  ap- 
plied to  the  primary  coil  is  used  (a)  to  overcome  the  resistance  Rf  of  the 


ALTERNATING-CURRENT  MACHINERY  241 

primary  coil;  (b)  to  overcome  the  electromotive  force  induced  in  the 
primary  coil  by  the  leakage  flux;  and  (c)  to  balance  the  electromotive 
force  induced  in  the  primary  coil  by  the  magnetic  flux  0$,  which 
passes  through  both  coils. 

The  part  (a)  of  OE'  is  equal  to  I'R',  and  it  is  in  phase  with  /'. 
The  part  (b)  of  OE'  is  equal  to  coLT,  and  it  is  90  degrees  ahead  of 
/'  in  phase.  The  part  (c)  of  OE'  is  represented  by  the  line  OA'. 
The  total  electroinotive  force  induced  in  the  secondary  coil  is  equal 

Ziff 

to  OA'  X  ^7.      This  electromotive  force  is  represented  by  the  line 
Z*   • 

OB.  A  portion  of  this  total  induced  electromotive  force  OB  is 
used  to  overcome  the  resistance  R"  of  the  secondary  coil;  and  the 
remainder  OE"  is  available  at  the  terminals  of  the  secondary  coil 
to  force  current  through  the  secondary  receiving  circuit. 

E" 
From  Fig.  207  it  is  evident  that  the  ratio  —  is  less  than  its 

Z"        OB 
ideal  value  —  (  =  77-77)    because  of  the  "resistance  loss"  I'R'  of 

electromotive  force  in  the  primary  coil,  because  of  the  "leakage  loss" 
w  L'  F  of  electromotive  force  in  the  primary  coil,  and  because  of  the 
"resistance  loss"  I"  R"  of  electromotive  force  in  the  secondary  coil. 

Performance.  With  Non-Inductive  Load.  When  a  transformer 
secondary  delivers  current  to  a  non-inductive  circuit,  then  01", 
Fig.  207,  is  parallel  to  OB,  and  01'  is  nearly  parallel  to  OE',  so  that 
R'l'  is  nearly  parallel  to  OE',  and  ajL'I'  is  nearly  at  right  angles  to 
OE'.  Therefore,  the  difference  in  value  between  OE'  and  OA'  is 
nearly  equal  to  I'R',  and  nearly  independent  of  a>L '/'.  Therefore,  the 
falling  off  of  the  secondary  voltage  E",  with  increase  of  load,  is  due 
almost  wholly  to  I'R'  and  to  I"R"  when  the  receiving  circuit  is  non-in- 
ductive, and  is  not  due,  to  any  perceptible  extent,  to  magnetic  leakage. 

With  Highly  Inductive  Load.  When  a  transformer  secondary 
delivers  current  to  a  highly  inductive  circuit,  then  01",  Fig.  207,  is 
nearly  at  right  angles  to  OB,  and  01'  is  nearly  at  right  angles  to  OE', 
so  that  I'R'  is  nearly  at  right  angles  to  OE';  cuLT  is  nearly  parallel 
to  OE';  and  further,  I"R"  is  nearly  at  right  angles  to  OB.  There- 
fore, the  difference  in  value  between  OE'  and  OA'  is  nearly  equal 
to  coLT,  and  nearly  independent  of  RT,  while  OB  is  nearly  equal  to 
OE".  Therefore,  the  falling  off  of  the  secondary  voltage  E",  with 


242  ALTERNATING-CURRENT  MACHINERY 

increase  of  load,  is  due  chiefly  to  ajLT,  that  is,  to  magnetic  leakage, 
when  the  receiving  circuit  is  highly  inductive,  and  is  not  due  to  any 
great  extent  to  coil  resistances. 

Example.  The  primary  of  a  certain  10-kilowatt  (1,000-volt  :  100-volt) 
transformer  has  a  resistance  of  1.5  ohms,  and  the  secondary  coil  has  a  resist- 
ance of  0.015  ohms.  The  secondary  coil  delivers  100  amperes  to  a  non-inductive 
receiving  circuit;  and,  ignoring  no-load  current,  the  primary  takes  10  amperes 
from  1,000-volt  mains.  The  IR  loss  of  electromotive  force  in  the  primary  coil 
is,  therefore,  1.5  ohms X 10  amperes  =  15  volts,  so  that  the  portion  OA',  Fig. 
207,  of  the  primary  applied  voltage  is  very  nearly  1,000  —  15=985  volts. 
Therefore,  the  total  electromotive  force  OB  induced  in  the  secondary  coil  is 

Z" 

— ^-X'985  volts  =  98. 5  volts.    The  IR  loss  of  electromotive  force  in  the  second- 

Z 

ary  coil  is  0.015  ohms  X 100  amperes  =  1.5  volts,  so  that  the  electromotive 
force  between  the  terminals  of  the  secondary  coil  is  98.5  volts  — 1.5  volts,  or 
97  volts.  In  this  case,  the  secondary  receiving  circuit  is  non-inductive,  and 
the  portion  a>L'Ir  of  the  primary  applied  voltage  is  nearly  at  right  angles  to 
E1 '.  This  loss  of  voltage  ajL'I'  has,  therefore,  no  appreciable  effect  in  lessening 
the  value  of  the  available  part  OA'  of  the  primary  applied  voltage  E'. 

The  leakage  reactance  wL'  of  the  above  transformer  is  5  ohms.  If  the 
secondary  coil  delivers  100  amperes  of  current  to  a  very  highly  inductive  re- 
ceiving circuit,  then  this  100  amperes  is  nearly  90  degrees  behind  OB,  Fig. 
207,  in  phase;  and  the  primary  current  of  10  amperes  is  nearly  90  degrees 
behind  OA'  in  phase.  Therefore,  the  leakage  voltage  loss  toL'I',  which  is  equal 
to  50  volts,  is  nearly  parallel  to  OA',  so  that  OA'  is  very  nearly  equal  to  1,000 

Z" 

volts— 50  volts,  or  950  volts,  and  E"  is  very  nearly  equal  to  —  X  950  volts,  or 

7i 

95  volts.  In  this  case,  I'R'  and  I"R"  are  nearly  at  right  angles  to  OA'  and 
OB,  and  these  resistance  losses  of  voltage  do  not  have  an  appreciable  effect 
in  lessening  the  secondary  terminal  voltage  E". 

The  above  discussion  of  the  effects  of  coil  resistances  and  of 
magnetic  leakage  shows  that  the  ratio  of  Ef  and  E"  is  very  nearly 

Zi' 

equal  to  its  ideal  value  -—when  the  primary  and  secondary  cur- 

^ 

rents  of  a  transformer  are  small,  that  is,  when  the  load  on  the  trans- 
former is  zero. 

TRANSFORMER  CONNECTIONS 

Parallel — Constant=Voltage  Transformers.  In  systems  of  dis- 
tribution where  alternating  currents  are  delivered  to  a  number  of 
units  (groups  of  lamps,  for  example)  all  at  constant  voltage,  each 
unit  (each  group  of  lamps)  is  supplied  from  the  secondary  of  a 
separate  and  distinct  transformer;  and  the  primaries  of  the  respective 
transformers  are  connected  in  parallel  across  the  constant-voltage 


ALTERNATING-CURRENT  MACHINERY  243 

mains  that  lead  out  from  the  supply  alternator.  This  arrangement 
is  shown  in  Fig.  208,  in  which  P,  P',  P",  etc.,  are  the  transformer 
primaries;  S,  Sf,  S",  etc.,  are  the  corresponding  secondaries;  and 
A,  A',  A",  etc.,  are  the  separate  receiving  units  or  groups  of  lamps. 
For  this  kind  of  service,  where  the  primary  of  a  transformer  is  sup- 
plied at  constant  voltage,  and  it  is  desired  that  the  transformer 
shall  deliver  current  to  a  receiving  unit  at  sensibly  constant  voltage 
irrespective  of  load,  i.  e.y  irrespective  of  the  number  of  lamps,  the 
transformer  must  be  designed  so  that  a  very  slight  decrease  of  in- 
duced electromotive  force  in  the  primary  will  permit  the  necessary 
current  to  flow  through  the  primary  coil.  This  requires  that  the 
coils  of  the  transformer  shall  have  as  little  resistance  as  possible, 
and  that  the  primary  and  secondary  coils  shall  be  wound  close 
together,  so  that  no  perceptible  portion  of  the  magnetic  flux  that  is 
forced  through  the  core  by  the  magnetizing  current  may  flow  out 
of  the  core  between  the  coils,  instead  of  passing  through  the  sec- 
ondary coil  as  well  as  through  the  primary  coil.  A  transformer 

IF"  I    5" 


•IESTEB1IB 


A'  A" 


Fig.  208.     Diagram  of  Connections  of  Constant- Voltage 
Transformers  in  Parallel 

specially  designed  to  realize  these  two  conditions  is  sometimes 
called  a  constant-potential  or  constant-voltage  transformer. 

A  constant-voltage  transformer  is  necessary  if  it  is  desired  to 
transform  a  given  voltage  in  a  determinate  ratio  so  as  to  be  able 
to  infer  the  value  of  the  given  voltage  from -the  measured  value  of 
the  transformed  voltage.  Specially  designed  transformers,  how- 
ever, are  not  always  necessary  for  this  purpose,  for  the  reason  that 
the  voltmeter  used  for  measuring  the  transformed  voltage  usually 
takes  very  little  current,  and  it  is  the  currents  in  a  transformer  that 
disturb  the  ideal  ratio  of  voltage  transformation. 

Multi-Coil  Type.  Most  commercial  transformers  are  now  made 
with  two  (or  more)  primary  coils,  and  with  two  (or  more)  secondary 
coils.  Each  of  the  primary  coils  of  such  a  transformer  may  be 


244 


ALTERNATING-CURRENT  MACHINERY 


adapted  to  direct  connection  to  1,100-volt  mains,  and  be  wound 
with  wire  large  enough  to  carry,  say,  10  amperes  without  undue  heat- 
ing. In  this  case,  if  these  two  primary  coils  are  properly  connected 
in  parallel  they  constitute  in  effect  a  single  primary  coil,  suited  for 
direct  connection  to  1,100-volt  mains,  and  capable  of  taking  20 
amperes  without  undue  heating.  On  the  other  hand,  if  these  two 


h 

m  t 

(!/  1 

a. 

g 

—  b 

If  III      tfffi! 


T  b 


Fig.  209.     Proper  Parallel 
Connections  for  Trans- 
formers 


Fig.  210.     Improper  Paral- 
lel Connections  for  Trans- 
formers 


primary  coils  are  properly  connected  in  series,  they  constitute  in 
effect  a  single  primary  coil  suited  for  direct  connection  to  2,200- volt 
mains,  and  capable  of  taking  10  amperes  of  current  without  undue 
heating.  Each  of  the  secondary  coils  of  such  a  transformer  may 
likewise  be  adapted  to  deliver  100  amperes  at  110  volts,  in  which 
case  the  two  secondaries,  if  properly  connected  in  parallel,  consti- 
tute in  effect  a  single  secondary  coil  adapted  to  deliver  200  amperes 
at  110  volts;  whereas,  if  the  two  secondaries  are  properly  connected 
in  series,  they  constitute  in  effect  a  single  secondary  adapted  to 
deliver  100  amperes  of  current  at  220  volts. 

Two  coils  of  a  transformer  (primary  or  secondary)  are  prop- 
erly connected  in  parallel  when  the  current  which  divides  between 
them  flows  around  the  core  in  the  same  direction  in  both  coils,  that 


tt» 


Fig.  211.     Proper  Series  Con- 
nections for  Transformers 


Fig.  212.     Improper  Series  Con- 
nections for  Transformers 


is,  so  that  both  coils  magnetize  the  core  in  the  same  direction.  Proper 
and  improper  connections  in  parallel  are  shown  diagrammatically  in 
Figs.  209  and  210.  See  also  "Polarity  Test,"  page  315. 

Two  coils  of  a  transformer  (primary  or  secondary)  are  properly 
connected  in  series  when  the  current  which  flows  through  them  flows 
around  the  core  in  the  same  direction  in  both  coils.  Proper  and 
improper  connections  in  series  are  shown  diagrammatically  in  Figs. 
211  and  212. 


ALTERNATING-CURRENT  MACHINERY 


245 


When  two  primaries  of  a  transformer  improperly  connected 
in  parallel  are  connected  to  the  supply  mains,  the  currents  in  the 
coils  oppose  each  other  in  their  magnetizing  action  on  the  core. 
The  result  is  that  the  core  is  not  perceptibly  magnetized;  but  little 
opposing  electromotive  force  is  induced  in  the  coils  (by  leakage 
flux);  and  the  two  improperly  connected  coils  constitute  a  short' 
circuit  when  they  are  connected  to  the  supply  mains. 

When  two  primaries  of  a  transformer,  improperly  connected 
in  series,  are  connected  to  the  supply  mains,  the  current  which 


Primary  Mams 


Fig.  213.     Proper  Transformer  Connections  to  a 
Three- Wire  System 

flows  through  the  two  coils  will  have  equal  and  opposite  magnet- 
izing actions  in  the  two  coils.  The  core  flux  will,  therefore,  be 
practically  zero,  and  no  counter-electromotive  force  will  be  induced 
in  the  windings  to  balance  the  applied  electromotive  force.  The 
flow  of  current  is,  therefore,  hindered  only  by  the  coil  resistances 
and  by  the  electromotive  forces  induced  by  the  leakage  flux.  The 
result  is  that  two  coils  improperly  connected  in  series  constitute  a 
short-circuit  when  they  are  connected  to  the  supply  mains. 

Two  secondary  coils  improperly  connected  in  parallel  give 
rise  to  short-circuit  conditions.  Two  secondary  coils  improperly 
connected  in  series  do  not  lead  to  short-circuit  conditions,  but  give 
zero  electromotive  force  between  their  terminals. 


246 


ALTERNATING-CURRENT  MACHINERY 


Edison  Three-Wire  System — Single-Phase.  The  Edison  three- 
wire  system,  extensively  used  in  direct-current  distribution,  is  com- 
monly used  in  alternating-current  distribution.  Fig.  213  shows  two 
transformers  properly  connected  for  supplying  current  to  a  three- 
wire  system;  and  Fig.  214  shows  two  transformers  improperly  con- 
nected for  supplying  current  to  a  three-wire  system.  In  the  proper 
connection,  the  middle  secondary  main  c  carries  only  the  difference 
of  the  currents  in  the  outside  mains  a  and  b;  and  in  the  improper 
connection,  the  current  in  the  middle  secondary  main  is  the  sum 
of  the  currents  in  the  outside  mains.  The  proper  connection  gives 


'/mnry  WCti 


r •  o  vo/rs -»t 

Fig.  214.     Improper  Transformer  Connections 
to  a  Three-Wire  System 

double  voltage  between  the  outside  secondary  mains  and  the  im- 
proper connection  gives  zero  voltage  between  outside  secondary 
mains. 

The  Edison  three-wire  system  must  not  be  confused  with  the 
two-phase  and  three-phase  three-wire  systems.  The  advantages  of 
the  Edison  three-wire  system  when  used  for  the  distribution  of 
single-phase  alternating  currents,  are  exactly  the  same  as  the  ad- 
vantages of  this  system  when  used  for  the  distribution  of  direct 
currents,  namely,  a  great  saving  in  the  copper  required  in  the  dis- 
tributing mains.  This  saving,  in  general,  amounts  to  five-eighths 
of  the  copper  that  would  be  required  for  a  two-wire  system  using 


ALTERNATING-CURRENT  MACHINERY 


247 


one-half  the  total  voltage  between  outside  mains.  As  explained  on 
page  5,  the  amount  of  copper  required  varies  inversely  as  the  square 
of  the  voltage  used. 

Single-phase  current  may  be  supplied  to  Edison  three-wire 
distributing  mains  by  a  single  transformer  having  two  secondary 
coils  properly  connected  in  series  to  the  outside  mains,  the  middle 
main  being  connected  to  the  junction  of  the  two  secondary  coils. 
An  "autotransformer"  or  "balance  coil"  having  a  tap  at  the  midcfte 
point  of  its  single  winding  is  often  used.  (See  page  252.) 


Fig.  215.     Connections  for  "Banked"  Transformers 

Banking  of  Transformers.  Two  service  mains  may  be  supplied 
with  current  by  two  or  more  transformers  with  their  primaries  con- 
nected in  parallel  between  the  supply  mains,  and  with  their  seconda- 
ries properly  connected  in  parallel  to  the  service  mains.  Fig.  215 
shows  two  transformers  properly  "banked"  so  as  to  supply  current 
to  the  two  service  mains  a  and  6.  In  many  large  transmission 
systems,  such  as  the  Niagara-Buffalo,  a  dozen  or  more  large  trans- 
formers may  be  banked  (on  each  phase)  for  step-up  or  step-down 
transformation . 

It  is  very  important  to  note,  however,  that  proper  connections 
alone  will  not  insure  satisfactory  parallel  operations.  Perfect  oper- 


248  ALTERNATING-CURRENT  MACHINERY 

ation  of  two  or  more  transformers  in  parallel  means  that  each  of 
the  separate  units  contributes  to  the  total  load  an  amount  of  power 
proportional  to  its  rated  output,  and  that  the  numerical  sum  of  the 
currents  in  the  separate  units  is  equal  to  the  line  (total)  current. 
To  secure  this  desirable  result,  two  conditions  must  be  fulfilled :  (a) 
the  ratio  of  primary  turns  to  secondary  turns  must  be  the  same  in 
all  the  units,  and  (b)  the  voltage  drop  from  no  load  to  full  load 
must  be  the  same,  both  in  magnitude  and  phase,  for  all  the  units. 

If  condition  (a)  is  not  fulfilled,  there  will  be  a  difference  at  all 
loads  in  the  secondary  voltages,  and  the  transformer  having  the 
highest  voltage  will  carry  the  largest  load.  Condition  (b)  is  ful- 
filled if  all  the  transformers  have  the  same  impedance  volts  (ZI) 
and  the  same  ratio  of  resistance  to  impedance.  If  the  transformers 
in  parallel  have  the  same  impedance  volts,  it  follows  that  the  mag- 
nitude of  the  current  furnished  by  each  will  be  inversely  as  its  im- 
pedance. If  further  they  have  the  same  ratio  of  reactance  to  resist- 
ance, it  follows  that  the  currents  delivered  by  each  will  have  the 
same  phase,  and  the  total  current  will  then  be  the  numerical  sum 
of  the  individual  currents. 

Given  two  or  more  transformers,  having  the  same  ratio  of  pri- 
mary to  secondary  turns  and  having  relatively  small  magnetizing 
currents,  to  be  banked  so  as  to  operate  in  parallel;  the  division  of 
the  total  load  between  them  depends  chiefly  upon  the  total  impe- 
dances between  the  bus  bars,  including  with  each  transformer  its 
connecting  wires  and  any  meters  or  relays  through  which  the  cur- 
rent may  pass.  In  any  case  where  transformers  banked  in  parallel 
do  not  divide  the  total  load  according  to  their  rated  capacities,  a 
proper  division  of  the  current  may  be  effected  by  increasing  the 
impedance  in  the  circuit  of  the  transformer  which  delivers  more 
than  its  share  of  the  total  current.  This  may  be  easily  done  by 
inserting  a  suitable  choke  coil  (a  coil  of  wire  wound  on  a  laminated 
iron  core)  in  series  with  either  its  primary  or  secondary  lead  wires. 

When  the  ratio  of  reactance  to  resistance  is  unequal  in  the 
transformers,  the  phases  of  the  secondary  currents  are  not  the  same, 
so  that  the  transformers  may  deliver  equal  currents,  and  still  not 
deliver  equal  amounts  of  power  to  the  circuit.  It  follows  that  a 
wattmeter  connected  with  its  series  (current)  coil  in  circuit  with  only 
one  of,  say,  two  transformers  of  equal  rating  will  not  measure 


ALTERNATING-CURRENT  MACHINERY  249 

half  of  the  total  power.     It  will  measure  more  than  half  or  less  than 
half  according  to  the  value  of  cos  6  in  the  expression  El  cos  0,  where 

reactance 

tan  0  =  — r-      - . 
resistance 

From  the  above  discussion  it  is  evident  that  special  care  must 
be  taken  in  banking  transformers  for  parallel  operation  to  see  that 
the  several  units  are  delivering  their  proper  share  of  the  total  cur- 
rent output.  Furthermore,  it  is  not  safe  to  assume  that  trans- 
formers even  of  the  same  rated  capacity  and  of  the  same  make  will 
share  a  given  load  equally  when  operated  in  parallel.  The  only 
safe  procedure  in  such  cases  is  to  measure  the  voltage,  current,  and 
watts  of  the  several  transformers. 

Series=Current  Transformers.  In  some  of  the  older  systems 
of  distribution  by  alternating  currents,  it  was  desired  that  each 


P  P1          P"         P"' 

vflg.pg.gj?/ ommuh vMgggg/ 


Fig.  216.     Connections  for  Transformers  in  Series 

receiving  unit  (arc  lamp,  for  example)  should  receive  a  constant 
current  equal  to  the  whole,  or  to  a  definite  fractional  part,  of  the 
constant  current  delivered  by  an  alternator.  This  condition  can 
be  realized  by  supplying  each  unit  or  group  of  units  from  the  sec- 
ondary of  a  separate  and  distinct  transformer,  the  primaries  of  all  the 
transformers  being  connected  in  series  as  shown  in  Fig.  216,  in  which 
P,  P',  P",  P"',  etc.,  are  the  primaries  of  the  respective  transformers; 
S,  S',  S",  S'",  etc.,  are  the  transformer  secondaries;  and  A,  A',  A" , 
A'"y  etc.,  are  the  receiving  units.  For  this  kind  of  service,  where 
the  primary  of  a  transformer  is  supplied  with  a  definite  current, 
and  it  is  desired  that  the  transformer  shall  deliver  to  a  receiving 
unit  a  current  which  is  equal  to  an  invariable  fractional  part  of  the 
primary  current,  irrespective  of  variations  of  resistance  in  this  unit, 
the  transformer  must  be  designed  to  take  as  small  a  magnetizing 
current  as  possible,  for,  according  to  the  discussion  on  page  235,  it 
is  the  magnetizing  current  that  disturbs  the  ideal  relation  of  primary 


250  ALTERNATING-CURRENT  MACHINERY 

to  secondary  current  in  a  transformer.  A  transformer  which  is 
specially  designed  to  realize  this  condition  is  sometimes  called  a 
current  transformer. 

The  transformer  used  in  connection  with  the  composite  field 
excitation  of  an  alternator,  as  shown  in  Fig.  112,  is  a  current  trans- 
former which  delivers  a  current  equal  to  a  definite  fraction  of  the 
current  output  of  the  alternator  to  the  rectifying  commutator. 

The  current  transformer  is  frequently  used  for  sending  current 
equal  to  a  definite  fractional  part  of  an  alternating  current  through 
an  ammeter  from  the  reading  of  which,  together  with  the  known 
ratio  of  current  transformation,  the  value  of  the  whole  alternating 
current  is  deduced. 

The  current  transformer  here  described  must  not  be  confused 
with  the  so-called  constant-current  transformer  which  receives  variable 


Fig.  217.     Diagram  of  Connections  for  Transformer  in  Series 
with  a  Number  of  Lamps 

current  from  a  constant  voltage  supply,  and  delivers  a  constant 
current  to  a  group  of  receiving  units  connected  in  series,  the  delivered 
current  being  constant  irrespective  of  increase  or  decrease  in  the 
number  of  receiving  units. 

The  connection  of  a  transformer  primary  in  series  in  a  circuit 
containing  many  other  elements  (lamps)  so  that  the  current  pass- 
ing through  the  primary  does  not  vary  much  with  the  varying  re- 
sistance of  the  circuit  to  which  the  secondary  of  the  transformer 
delivers  current,  gives  rise  to  actions  which  are  not  very  familiar 
to  electrical  engineers,  for  the  reason  that  this  arrangement  is  now 
seldom  used  in  practice.  The  actions,  however,  which  are  inter- 
esting, are  as  follows: 

Fig.  217  represents  a  transformer  primary  P  connected  in 
series  in  a  circuit  containing  many  elements  e',  e",  e'",  etc.,  (lamps). 


ALTERNATING-CURRENT  MACHINERY  251 

The  action  will  be  described  in  two  steps,  viz,  (a) "on  the  assumption 
that  the  magnetizing  current  of  the  transformer  is  always  negligi- 
ble, and  (b)  without  the  aid  of  this  simplifying  assumption. 

(a)  If  the   magnetizing   current   is   always   negligibly   small, 
then  the  current  in  A  is  equal  to  a  fixed  fractional  part  of  the  sensibly 
constant  current  in  the  main  circuit,  so  that  any  increase  of  the 
resistance  or  reactance  of  A  must  be  accompanied  by  a  correspond- 
ing increase  of  the  voltage  E",  which  is  pushing  current  through  ~A; 
and  this  must  be  accompanied  by  a  corresponding  increase  of  the 
voltage  Ef  between  the  terminals  of  the  primary  coil  P.    Thus,  if 
A  has  zero  resistance  and  zero  reactance,  then  E"  is  zero,  and  E'  is 
zero.    That  is,  the  current  in  the  main  circuit  flows  through  P  with- 
out any  opposition  at  all,  just  as  if  P  were  a  connection  of  zero  re- 
sistance. 

If  the  resistance  or  reactance  of  A  is  increased,  E"  (and  also  E') 
must  increase,  which  means  that  the  current  in  the  main  circuit 
encounters  greater  and  greater  opposition  in  flowing  through  P. 

If  the  circuit  of  A  is  opened  (infinite  resistance),  then,  on  the 
above  assumption  of  negligible  magnetizing  current,  the  opposition 
to  the  flow  of  current  in  P  becomes  infinite,  so  that  breaking  the 
circuit  of  A  is  equivalent  to  breaking  the  main  circuit. 

(b)  As  a  matter  of  fact,  as  the  resistance  or  reactance  of  A  is 
increased,  causing  an  increase  of  E"  and  E',  the  magnetism  of  the 
transformer  core  must  increase  proportionally  with  E"  and  E'  in 
order  that  these  increased  voltages  may  actually  be  induced  in  the 
transformer  coils;  and  this  increase  of  magnetism  of  the  core  requires 
more  and  more  magnetizing  current.*     The  magnetizing  current, 
therefore,  is  not  always  negligible  irrespective  of  the  resistance  of  A. 
In  fact,  when  the  resistance  of  A  is  infinite  (open  circuit),  there  is 
no  secondary  current;  all  the  current  in  the  coil  P  is  magnetizing 
current;  and  the  voltage  E',  which  opposes  the  flow  of  current 
through  P,  rises  only  to  that  value  which  corresponds  to  the  degree 
of  magnetism  of  the  core  that  can  be  produced  by  the  magnetizing 
action  of  the  whole  primary  current.    The  transformer  then  becomes 
simply  a  choke  coil. 

Autotransformer.     Given   a   source   of  supply   of  alternating 

*It  must  be  remembered  that  the  magnetizing  current  is  that  part  of  the  primary 
current  whose  magnetizing  action  is  not  balanced  or  annulled  by  the  secondary  current  as 
explained  on  page  235. 


252  ALTERNATING-CURRENT  MACHINERY 

current.  This  current  may  be  delivered  to  a  receiving  unit  in  three 
ways: 

(a)  By  connecting  the  unit  directly  to  the  supply  mains. 

(b)  By  connecting  the  unit  to  the  secondary  of  a  transformer  of  which 
the  primary  is  connected  to  the  supply  mains. 

(c)  By  a  combination  of  methods  (a)  and  (b). 

The  combination  method  may  be  realized  with  any  ordinary  trans- 
former; which,  when  so  used,  is  called  an  autotransformer,  o*-  com- 
pensator. 

NOTE.  It  will  appear  in  the  following  discussion  that  when  the  voltage 
of  the  supply  differs  but  little  from  the  desired  service  voltage,  the  combina- 
tion method  is  much  preferable  to  the  method  in  which  the  transformer  alone 
is  used,  because  of  the  fact  that  a  smaller  transformer  suffices,  and  because 
the  combination  method  involves  less  energy  loss.  This  combination  or  auto- 
transformer  method  is  quite  simple  of  treatment  when  attention  is  confined 
to  a  particular  case;  but  it  is  complicated  when  attempt  is  made  to  give  it  a 
general  discussion,  that  is,  when  attempt  is  made  to  discuss  all  the  theoretically 
possible  ways  in  which  4a  given  ordinary  transformer  may  be  used  as  an  auto- 
transformer. 

In  Fig.  218  A  and  B  are  alternating-current  supply  mains, 
between  which  the  voltage  is,  say  100;  C  and  D  are  service  mains, 

to  which  it  is  desired  to  deliver 
alternating  current  at  90  or  110 
volts;  P  and  S  are  the  primary 
and  secondary  coils  of  an  ordinary 
transformer.  The  primary  P  is 
connected  to  A  and  B  as  shown. 
Fig.  218.  Connections  for  Auto  step-Up  The  secondarv  S  has  one-tenth  as 

Transformer  * 

many  turns  as   P,   therefore,   the 

voltage  induced  in  S  is  one-tenth  of  the  voltage  acting  on  P,  or  10 
volts.  Let  the  dotted  arrows  represent  the  directions  of  the  induced 
electromotive  forces  in  the  two  coils  at  a  given  instant.  Then  the 
long  heavy  arrow  will  represent  the  direction  of  the  voltage  between 
the  mains  at  the  same  instant,  inasmuch  as  the  induced  voltage  in 
the  primary  coil  of  a  transformer  is  always  opposed  to  the  supply 
voltage. 

Auto  Step-Up  Transformation.  The  10  volts  induced  in  the 
coils,  Fig.  218,  will  help  push  current  into  the  service  mains  if  we 
connect  from  supply  main  A,  out  of  which  the  current  at  the  given 
instant  is  tending  to  flow,  to  terminal  g  of  coil  S;  connect  from 


ALTERNATING-CURRENT  MACHINERY  253 

terminal  /  to  service  main  C;  and  connect  from  service  main  D  to 
supply  main  B,  as  shown  by  the  dotted  lines.  In  this  case,  the  vol- 
tage induced  in  the  coil  S  is  added  to  the  supply  voltage,  and  the 
service  voltage  is,  therefore,  110  volts. 

Auto  Step-Down  Transformation.  The  10  volts  induced  in  the 
coil  S  will  oppose  the  flow  of  current  into  the  service  mains  if 
we  connect  from  supply  main  A  to  terminal/,  connect  from  terminal 
g  to  service  main  C,  and  connect  from  service  main  D  to  supply 
main  B,  as  shown  by  the  dotted  lines  in  Fig.  219.  In  this  case  the 
induced  voltage  in  the  coil  S  is  subtracted  from  the  supply  voltage, 
and  the  service  voltage  is  now  90. 

Current  Relations.  In  Figs.  218  and  219  the  directions  of  the 
induced  voltages  in  P  and  S  are  shown  by  the  dotted  arrows.  These 
induced  voltages  are  in  the  same  direction  in  the  two  coils.  The 
currents  in  the  two  coils  of  a  transformer  are,  on  the  other  hand, 
always  in  opposite  directions,  inasmuch  as  they  balance  the  mag- 
netizing action  of  each  other.  Suppose  for  the  sake  of  concreteness 
that  10  amperes  are  delivered  to  the  service  mains.  Then  we  have 
the  following  relations: 

(a)  Ten  amperes  flow  through  S, 
Fig.  218,  in  the  same  direction  as  the  in- 
duced electromotive  force  of  ten  volts,  so 
that  one  ampere  (one-tenth  as  much  cur- 
rent, since  there  are  ten  times  as  many 
turns  in  P  as  in  S)  flows  through  P  in 
opposition  to  the    induced   or   counter- 
electromotive  force  0f  100  volts.      There-  Fig.  219.     Connections  for  an  Auto 
fore,    the    COil    P    takes    100  watts    from  Step-Down  Transformer 

the  supply  mains,  which  power  is  trans- 
ferred to  the  coil  S  by  ordinary  transformer  action,  whence  it  is  given  out  (ten 
volts  pushing  ten  amperes)  in  assisting  the  flow  of  current  to  the  service  mains. 
The  total  power  delivered  to  CD  is  evidently  1,100  watts  (10  amperes  at  110 
volts),  and  of  course  the  total  power  taken  from  the  supply  mains  is  1,100 
watts  (11  amperes  at  100  volts). 

(b)  Ten  amperes  flow  through  S,  Fig.  219,  in  a  direction  opposite  to 
the  ten  volts  of  induced  electromotive  force,  so  that  one  ampere  flows  through 
P  in  the  same  direction  as  the  induced  electromotive  force  of  100  volts.     There- 
fore, of  the  1,000  watts  delivered  by  the  supply  mains  in  forcing  the  ten  am- 
peres through  S  and  through  the  service  mains,  100  watts  are  delivered  to  the 
coil  S,  and  900  watts  are  delivered  to  the  service  mains  (10  amperes  at  90 
volts).     The  100  watts  delivered  to  S  are  transferred  by  transformer  action 
to  the  coil  P,  and  delivered  by  P  back  to  the  supply  mains. 

It  is  to  be  particularly  noted  that  only  100  watts  are  involved  in  the 


254 


ALTERNATING-CURRENT  MACHINERY 


above  as  genuine  transformer  action,  although  900  or  1,000  watts  are  actually 
delivered  to  the  service  mains. 

The  autotransformer  is  used  commercially  for  many  purposes. 
A  common  use  is  as  a  "balance  coil"  in  a  three-wire  distribution  from 
a  two-wire  supply,  arid  it  is  also  used  as  a  balancer  in  connection 
with  rotary  converters  for  supplying  a  three-wire  direct-current 
service  as  explained  on  page  355.  Compensators  are  also  used  for 
the  operation  of  low-voltage  tungsten  lamps  in  parallel  for  house 
lighting,  for  electric  signs,  for  arc  lamps,  as  voltage  regulators  foir 
mercury-vapor  rectifiers,  as  explained  on  page  324.  Autotrans- 
formers  are  also  widely  used  as  "starting  compensators"  for  alter- 
nating-current motors.  In  such  cases  they  supply  a  reduced  voltage 
(usually  half  voltage)  to  the  motor  circuits  while  the  armature  is 
accelerating  from  rest.  Each  autotransformer  is  usually  provided 
with  several  taps  so  that  a  number  of  low  voltages  may  be  obtained 
at  will. 

Autotransformers  are  used  for  supplying  a  varying  voltage  to 
single-phase  commutator  motors  used  on  electric  cars  and  locomo- 
tives, and  thus  for  controlling  the  speed  of  the  motors. 

TRANSFORMERS  IN  POLYPHASE  SYSTEMS 

A  polyphase  transmission  system  is  essentially  the  utilization 
of  two  or  three  entirely  separate  and  distinct  single-phase  transmis- 


P"     S" 


Fig.  220.     Transformer  Con- 
nections on  a  Two-Phase 
System 


o 
o 
o 


S" 

Fig.  221.    Transformer  Con- 
nections on  a  Three-Wire 
Two-Phase  System 


sion  systems  of  which  the  separate  and  distinct  electromotive  forces 
or  currents  are  maintained  in  definite  phase  relations  with  each  other  by 
mechanical  connections  in  the  generator.  Step-up  or  step-down  trans- 
formation in  a  polyphase  system  is  accomplished,  in  general,  by  a 
separate  and  distinct  transformer  of  the  ordinary  type  for  each  phase. 


ALTERNATING-CURRENT  MACHINERY  255 

Two=Phase  System.     Fig.  220  shows  a  two-phase  system  in 
which  the  current  of  each  phase  A  and  B  is  transmitted  over  an 


Fig.  222. 


Connections  for  Three  Transformers  with  Primaries  Con- 
nected to  Three-Wire  Three-Phase  Mains 


entirely  independent  circuit,  and  the  electromotive  force  of  each 
phase  is  stepped  down  (or  up)  by  an  ordinary  transformer  P'/S'  and 
P"S",  respectively.  Fig.  221  shows  what  is  called  the  three-wire, 
two- phase  system,  in  which  one  line  wire  is  used  as  a  common  return 
wire  for  both  phases,  and  where  two  ordinary  transformers  P'S'  and 
P"S"  are  used  for  stepping  the  voltage  down  (or  up).  These  two 
figures  contain  all  that  is  essential  in  the  step-down  or  step-up  trans- 
formation of  a  two-phase  system. 


Fig.  223.  System  Shown  in  Fig.  222  with  One  Transformer  Removed 

Three=Phase  System.    The  usual  transmission  line  for  a  three- 
phase  system  consists  of  three  wires,  each  wire  being  in  effect  a  com- 


256 


ALTERNATING-CURRENT  MACHINERY 


Fig.  224. 


Diagrammatic  View  of  Two  Separate  Trans- 
formers for  Step-Up  or  Step-Down  Transforma- 
tion— Two-Phase  System 


mon  return  for  currents  that  pass  out  over  the  other  two.  In  this 
case,  the  usual  arrangement  for  step-down  (or  step-up)  transforma- 
tion is  to  connect  the 
three  primaries  of  three 
ordinary  transformers 
to  the  supply  mains, 
using  either  Y  or  A  con- 
nections; and  to  connect 
the  three  secondaries  to 
the  service  mains,  using 
either  Y  or  A  connec- 
tions. Furthermore,  the 
primaries  may  be  Y-con- 
nected  and  the  secondaries  A-connected,  or  vice  versd.  The  A  connec- 
tions of  both  primaries  and  secondaries  are  preferred  in  practice,  inas- 
much as  with  this  [arrangement  the  complete  three-phase,  step-down 
(or  step-up)  transformation  is  still  effected  even  though  one  trans- 
former may  be  entirely  disconnected  because  of  burn-out  or  break- 
down. In  this  case  the  two  remaining  transformers  are  said  to  be  con- 
nected in  V .  In  such  a  case,  however,  the  two  remaining  transformers 
do  not  have  two-thirds  of  the  transforming  capacity  of  all  three,  but 
only  TVir  of  |  =  0.567  as  much,  or  a  little  over  one-half  the  capacity. 
Fig.  222  shows  three  ordinary  transformers  P'S',  P"S",  and 
P'"S'",  with  their  primaries  A-connected  to  three-wire,  three-phase 
supply  mains  1,  2,  and  3;  and  with  their  secondaries  A-connected 
to  three-wire,  three-phase  service  mains  a,  6,  and  c. 

Fig.  223  shows  the  arrange- 
ment of  Fig.  222  with  one  of 
the  transformers  P'"S'"omit- 
ted  (any  one  of  the  three  may 
be  omitted),  thus  giving  the 
V  connection.  This  arrange- 
ment is  operative  for  three- 
phase,  three-wire  step-up  or 
step-down  transformation,  ex- 
cept that  its  power  capacity  is 
only  about  57  per  cent  of  the  power  capacity  of  the  arrangement  in 
which  three  similar  transformers  are  used. 


pi 


II 


Fig.  225.     Two  Transformers  for  Two-Phase  Sys- 
tem with  Common  Magnetic  Return  Circuit 


ALTERNATING-CURRENT  MACHINERY 


257 


Transformers  with  Compound  Magnetic  Circuits.    Let  A  and 

B,  Fig.  224,  be  the  two  separate  transformers  to  be  used  for  step-up 
or  step-down  transfor- 
mation on  a  two-phase 
system.  Each  iron  core 
may  have  its  own  re- 
turn circuit  for  the  mag- 
netic flux;  or  a  single 
magnetic  return  C  may 
be  used  for  the  two,  as 

Shown   in   Fig.   225.       In  Fig.  226.     Three  Transformers  for  Three-Phase  System 

,,        i     .  i       -i    A  with  Common  Magnetic  Return  Circuit 

the  latter  case  only  1.4 

as  much  iron  need  be  used  for  the  common  return  as  would  have  to 

be  used  for  each  single  magnetic  return. 

Similarly,*  three  transformers  A,  B,  and  C,  Fig.  226,  used  for 
step-up  or  step-down;  transformation  on  a  three-phase  system,  may 
be  combined  magnetically  so  that  each  transformer  core  is  the 
magnetic  return  for  the  other  two,  as  shown.  Commercial  examples 
of  polyphase  transformers  are  described  on  pages  292  and  293. 

Phase  Transformation.  If  a  two-phase*  supply  is  connected 
to  the  similar  primaries  of  two  transformers,  an  electromotive  force  of 
any  desired  value  and  of  any  desired  phase  may  be  produced.  Fig.  227 
shows  the  two-phase  supply  mains  connected  to  the  similar  primaries 
P'  and  P"  of  two  separate  transformers.  On  core  A  is  wound  a 
secondary  coil  a,  and  on  core  B  is  wound  a  secondary  coil  b;  these 
two  secondary  coils 
are  connected  in  I  P' 

series,  and  the  desired 
electromotive  force  E 
is  produced  by  the 
two  secondary  coils  in 
conjunction.  In  order 
that  the  desired  elec- 
tromotive force  E  may  Fig.  227.  Connections  for  Phase  Transformation  from 
.  ..  '  Two-Phase  Supply  Mains 

be  produced  by  coils 

a  and  b  jointly,  the  following  conditions  must  be  fulfilled: 


*Or    three-phase   supply.      The  discussion  of  phase  transformation  is,  however,  much 
simpler  with  a  two-phase  supply. 


258 


ALTERNATING-CURRENT  MACHINERY 


Fig.  228.     Vector  Diagram  of 
E.M.F.  Relations  for  Fig.  227 


Let  the  vectors  A  and  B  in  the  clock  diagram,  Fig.  228,  rep- 
resent the  two  two-phase  electromotive  forces;  and  let  E  represent 

the  desired  electromotive  force.  Then  coil 
a,  Fig.  227,  must  be  wound  with  a  sufficient 
number  of  turns  of  wire  to  produce  the 
component  a  of  E;  and  coil  b  must  be 
wound  with  a  sufficient  number  of  turns  of 
wire  to  produce  the  component  b  of  E.  See 
Fig.  228. 

If  it  is  desired  that  the  resultant  elec- 
tromotive force  E  be  in  the  second,  third,  or  fourth  quadrant, 
the  coil  a  or  the  coil  b,  or  both  coils  a  and  b  must  be  reversed 
as  indicated  in  Fig.  229.  It,  therefore,  follows  that  by  cor- 
rectly proportioning  and  connecting  the  coils  a  and  b,  any 
desired  value  and  position  of  the  resultant  electromotive  E  may  be 
produced. 

The  general  two-phase,  three-phase  transformer  consists  of 
two  separate  transformers  with  similar  primary  coils  Pr  and  P" 
which  are  connected  to  the  respective  phases  of  the  two-phase 
supply  mains,  as  shown  in  Fig.  227;  and  each  of  the  three-phase 
electromotive  forces  is  produced  by  a  pair  of  properly  proportioned 
and  properly  connected  secondary  coils  (one  coil  on  each  trans- 
former) connected  in  series.  Each  pair  of  secondary  coils  constitutes 
one  unit  of  the  three-phase  system,  and  these  three  units  may  be 
either  Y-connected  or  A-connected  to  three-wire,  three-phase  service 
mains. 

The    general    two-phase,   three-phase    transformer    is    greatly 

simplified  if  we  choose  to 
have  one  of  the  three-phase 
electromotive  forces  in  phase 
with  one  of  the  two-phase 
electromotive  forces.  Thus, 
if  E,  Fig.  228,  is  in  phase 
with  B,  then  E  may  be 
produced  by  a  single  sec- 
ondary coil  on  core  B, 
instead  of  being  produced  by  a  pair  of  secondary  coils,  one  on 
each  core. 


Fig.  229.     Vector  Diagram  of  E.M.F.  Relations 
for  E  in  any  Quadrant 


ALTERNATING-CURRENT  MACHINERY 


259 


Fig.  230.     Vector  Diagram  of 

E.M.F.  Relations  in  Three 

Secondaries  of  Scott 

Transformer 


'000000 

a 


Fig.  231.      Diagram  of  Di- 
vided Secondary  in  Scott 
^Transformer 


Scott  Transformer.  The  two-phase,  three-phase  transformer  per- 
mits of  still  further  simplification  if  the  A-connection  of  the  three-phase 
units  is  excluded,  that  is,  if  the  three-phase 
units  are  to  be  adapted  only  for  Y-connection 
to  the  three-phase  mains.  This  ultimate  sim- 
plification is  realized  in  the  Scott  transformer. 

The  transformer  shown  in  Fig.  227 
may  be  converted  into  a  Scott  transformer 
by  replacing  its  windings  with  the  wind- 
ings shown  in  Fig.  234.  The  two  similar 
primary  coils  A  and  B,  which  are  connected 
to  the  respective  phases  of  a  two-phase  system,  are  placed  on  cores 
A  and  B,  respectively.  Coils  b,  a,  and  c  are  the  three  secondaries 

_  which    are   Y-connected    to    the    three-phase 

mains,  b  is  placed  on  core  B  and  gives  the 
electromotive  force  b,  Fig.  230.  It  is  to  be 
noticed  that  coils  a  and  c  are  formed  by 
bringing  out  a  connection  at  the  mid-point 
of  one  large  coil,  Fig.  231.  a  and  c  are  placed  on  core  A,  Fig.  227, 
and  give  the  electromotive  forces  a  and  c,  Fig.  230. 

The  number  of  turns  on  a 
and  c  are  equal.  The  number  of 
turns  on  b  are  made  to  equal  V  3 
times  the  turns  on  either  a  or  c, 
i.e.,\ia  and  c  each  have  50  turns,  b 
will  have  approximately  87  turns. 

The  points  1,  2,  and  3,  Fig. 
230,  are  at  the  angles  of  an  equilat- 
eral triangle.  The  point  0  repre- 
sents the  common  junction  point 
of  the  three  terminals,  one  from 
each  of  the  coils  a,  6,  and  c.  The 
other  terminals  of  these  coils  are 
connected  to  the  three-phase 
mains  represented  by  the  figures 
1,  2,  and  3,  Figs.  230  and  234. 

Review  page  124,  Book  II,  to  get  a  clear  idea  of  the  electromotive 
force  and  current  relations  in  three-phase  systems  Y-connected, 


Fig.  232.     Vector  Diagram  of  E.M.F. 
Relations  in  Scott  Transformer 


260 


ALTERNATING-CURRENT  MACHINERY 


JWQQMQfl 


From  these  relations  and  keeping  in  mind  the  ratios  of  the  wind- 
ings of  the  coils  in  Fig.  234,  it  may  be  seen  that  the  electromotive 
force  a  helps  to  push  current  in  a  receiving  circuit  from  main  1  to 
main  2;  while  electromotive  force  b  opposes  a 
in  this  respect.  Therefore,  the  electromotive 
force  from  main  1  to  main  2  is  a—  b,  as  shown 
in  Fig.  232.  Similarly  the  electromotive  force 
b  helps  to  push  current  in  a  receiving  circuit 
from  main  2  to  main  3,  while  electromotive 
force  c  opposes  b  in  this  respect.  Therefore, 
the  electromotive  force  from  main  2  to  main 
3  is  b—c,  as  shown  in  Fig.  232.  Lastly  the 
electromotive  force  c  helps  to  push  current  in  a  receiving  circuit 
from  main  3  to  main  1,  while  the  electromotive  force  a  opposes  c  in 
this  respect.  Therefore,  the  electromotive  force  from  main  3  to 
main  1  is  c— a,  as  shown  in  Fig.  232. 

An  inspection  of  Fig.  232  will  show  that  a—b=b—c=c—a. 
Therefore,  if  E  is  allowed  to  represent  the  value  of  each,  this  dis- 
cussion shows  that  the  electromotive  forces  between  mains  1  to  2, 
2  to  5,  and  3  to  1  have  the  common  value  E  volts,  and  are  120  degrees 
apart  in  phase,  provided 


Fig.  233.     Y  Connections 

for  Three  Secondaries  of 

.  Scott  Transformer  for 

Two-Phase  System 


and 


b  =  E  cos  30° 


=Esin  30C 


But  the  ratios  of  the  windings  were  so  proportioned  that  this  is  true. 

Hence  the  e.  m.  f.  s  are  120  degrees 
apart  and  result  in  a  balanced 
three-phase  circuit. 

A  clear  idea  of  the  Scott  trans- 
former may  now  be  obtained  as 
follows:  Two  similar  cores  have 
similar  primary  coils,  which  are 
connected  to  the  respective  phases 
of  a  two-phase  system.  One  of 

these    cores    has    a    secondary    winding   b,   one   end    of   which  is 
connected  to  one  of  the  three-phase  mains  (main  2,  as  shown  in 


Fig.  234. 


Complete  Connections  of  Scott 
Transformer 


ALTERNATING-CURRENT  MACHINERY  261 

~\ 

Fig.  233),  and  the  other  end  of  which  is  connected  to  the  middle 
point  of  the  secondary  winding  a  c,  which  is  wound  on  the  other  core. 
The  terminals  of  the  winding  a  c  are  connected  to  the  remaining 
two  of  the  three-phase  mains  (mains  1  and  3,  as  shown  in  Fig.  233). 

2 
The  entire   secondary   winding  ac  has    — =    times     as     many 

V       O 

turns  of  wire  as  the  coil  b,  that  is,  1.16  times  as  many  turns._ 
The  complete  connections  of  the  Scott  transformer  are  shown 
in  Fig.  234. 

The  three-phase  system  requires  less  line  copper  than  either  the 
single-phase  or  the  two-phase  system  to  transmit  a  given  amount 
of  power  with  a  given  line  voltage  and  with  a  given  loss.  Hence, 
for  the  long-distance  transmission  of  electric  power,  the  three- 
phase  system  is  universally  adopted  in  this  country.  For  the  local 
distribution  of  electric  power,  on  the  other  hand,  the  two-phase 
system  offers  certain  advantages.  It  is  often  the  case,  therefore, 
that  two-phase  alternators  are  used  to  generate  alternating  currents 
at  a  central  station,  and  that  two-phase  currents  are  used  for  power 
and  lighting  purposes  in  the  neighborhood  of  the  station.  When, 
however,  power  is  to  be  transmitted  to  points  fifteen  or  more  miles 
distant,  it  becomes  desirable,  as  explained  above,  to  use  the  three- 
phase  system.  It  is  in  such  cases,  especially,  that  phase  tranforma- 
tion  is  used.  The  Niagara-Buffalo  transmission  is  the  earliest  as  well 
as  one  of  the  most  extensive  examples  of  this  practice.  Power  is 
generated  by  large  two-phase  alternators  at  2,200  volts.  A  large 
part  of  this  power  is  distributed  to  factories  and  chemical  works  in 
the  vicinity  of  the  central  power  plant.  A  large  amount  of  power 
also  is  transmitted  to  Buffalo,  a  distance  of  eighteen  miles,  by  means 
of  three-phase  alternating  currents  derived  from  the  two-phase  alter- 
nators by  two-phase,  three-phase  transformation.  Scott  trans- 
formers are  used;  and  the  two-phase  currents  at  a  voltage  of  2,200 
are  stepped  up  to  22,000  volts,  and  at  the  same  time  are  trans- 
formed to  three-phase  currents.  At  Buffalo,  the  three-phase,  high- 
voltage  currents  are  stepped  down  to  about  2,200  volts,  and  are 
transformed  back  into  two-phase  currents,  also  by  Scott  three-phase 
two-phase  transformers.  The  2,200-volt  two-phase  currents  are  then 
distributed  by  feeders  and  service  mains  for  lighting  and  power 
purposes  throughout  the  city. 


262  ALTERNATING-CURRENT  MACHINERY 

PRACTICAL  CONSIDERATIONS 

Transformer  Losses.  The  power  output  of  a  transformer  is  less 
than  its  power  intake  because  of  the  losses  in  the  transformer.  These 
losses  are:  (a)  the  iron  or  core  losses  due  to  eddy  currents  and  hys- 
teresis ;  and  (b)  the  copper  losses  due  to  the  resistances  of  the  primary 
and  secondary  coils. 

(a)  Iron  Losses.  The  iron  losses  are  practically  the  same  in 
amount  at  all  loads.  They  depend  upon  the  frequency  and  range 
of  the  flux  density  B,  upon  the  quality  and  volume  of  the  iron,  and 
upon  the  thickness  of  the  laminations.  Dr.  C.  P.  Steinmetz  has 
found  by  exhaustive  experiments  that  for  ordinary  sheet  steel  the 
hysteresis  loss  may  be  expressed  in  watts  as 

jrfc=?F/B"X10-7.  (35) 

in  which  /  is  the  frequency  in  cycles  per  second ;  B  is  the  maximum 
flux  density  in  the  iron  core,  in  lines  per  square  centimeter;  V  is  the 
volume  of  the  iron,  in  cubic  centimeters;  and  y  is  a  coefficient  de- 
pending upon  the  magnetic  quality  of  the  iron.  For  plain  electrical 
sheet  steel  used  for  transformer  cores  the  value  of  ^  is  about  0.0021. 
For  silicon-steel,  now  much  used  in  constructing  the  cores  of  trans- 
formers, the  value  of  7?  may  be  taken  as  about  0.00093. 
The  eddy  current  loss,  in  watts,  is 

JFe  =  6F/2*2B2X10-7  (36) 

in  which  t  is  the  thickness  of  the  laminations,  in  centimeters;  and 
b  is  a  constant  depending  upon  the  specific  electrical  resistance  of 
the  steel.  For  ordinary  sheet  steel  the  value  of  b  is  1.65X10"11, 
and  for  silicon-steel  0.57  X  10" n.  Insufficient  insulation  between 
laminations  causes  excessive  eddy  current  loss,  and  results  in  a  much 
higher  loss  than  equation  (36)  indicates.  The  equation  is  derived 
on  the  condition  of  perfect  insulation  between  laminations,  a  con- 
dition which  is  hardly  ever  realized  in  practice. 

Equations  (35)  and  (36)  may  be  used  for  calculating  the  ap- 
proximate hysteresis  and  eddy-current  losses  in  any  mass  of  laminated 
iron  subjected  to  periodic  magnetization,  such  as  alternator  armatures 
and  the  rotor  and  stator  iron  in  an  induction  motor,  but  the  losses 
thus  calculated  are  usually  smaller  than  the  actual  losses.  It  is 
preferable  when  possible  to  find  by  a  wattmeter  test  the  actual 
total  core  loss  per  pound  of  steel  at  different  flux  densities  and 


ALTERNATING-CURRENT  MACHINERY 


263 


TABLE  VII 
Transformer  Efficiencies,  Losses,  Etc. 


KV-A. 

WATTS 
Loss 

PER  CENT  EFFICIENCY 

PER  CENT  REGULATION 

*  Exciting 
Current 

Iron 

Cop- 
per 

Full 
Load 

H  Load 

Yz  Load 

M  Load 

TF% 

p°% 

??% 

P°% 

* 

15 

13 

94.7 

94.4 

93.2 

88.7 

2.62 

3.21 

3.28 

3.16 

8.0 

1 

20 

24 

95.8 

95.7 

95.1 

92.0 

2.42 

3.03 

3.12 

3.04 

5.5 

12L 

25 

35 

96.0 

96.0 

95.5 

92.7 

2.36 

2.96 

3.07 

3.00 

4.0 

2 

30 

42 

96.5 

96.5 

96.2 

93.8 

2.12 

2.76 

2.88 

2.86 

3.6 

2i 

33 

51 

96.8 

96.8 

96.5 

94.5 

2.08 

2.71 

2.83 

2.83 

3.3 

3 

34 

64 

96.8 

97.0 

96.8 

95.2 

2.16 

2.79 

2.91 

2.88 

3.0 

4 

40 

75 

97.2 

97.3 

97.1 

95.7 

1.90 

2.77 

3.00 

3.12 

2.5 

5 

45 

93 

97.3 

97.5 

97.3 

96.1 

1.90 

2.76 

2.99 

3.11 

2.3 

7i 

62 

125 

97.6 

97.7 

97.6 

96.4 

1.70 

2.60 

2.84 

3.00 

2.2 

10 

80 

148 

97.8 

97.9 

97.7 

96.5 

1.51 

2.42 

2.68 

2.89 

1.9 

15 

105 

212 

97.9 

98.0 

97.9 

97.0 

1.44 

2.36 

2.63 

2.85 

1.6 

20 
25 

131 

268 

98.0 

98.1 

98.0 

97.1 

1.39 

2.51 

2.87 

3.21 

1.5 

147 

319 

98.2 

98.3 

98.2 

97.4 

1.33 

2.45 

2.82 

3.17 

1.3 

30 

163 

374 

98.2 

98.4 

98.3 

97.6 

1.32 

2.45 

2.82 

3.16 

1.2 

37i 

197 

433 

98.3 

98.4 

98.4 

97.7 

1.20 

2.34 

2.72 

3.09 

1.2 

50 

240 

550 

98.4 

98.6 

98.5 

97.9 

1.15 

2.29 

2.68 

3.07 

1.0 

*In  per  cent  of  full  load  current. 

frequencies.  Curves  may  then  be  plotted  one  for  each  fre- 
quency, using  total  core  loss  in  watts  per  pound  as  abscissas,  and 
B  as  ordinates. 

(b)     Copper  Losses.    The  copper  losses,  in  watts,  are 

W0=RfI'*+R"I"\  (37) 

This  loss  is  nearly  zero  when  the  transformer  is  not  loaded;  it  in- 
creases with  the  square  of  the  current;  and  becomes  excessive  when 
the  transformer  is  greatly  overloaded. 

Transformer  Efficiency.  The  ratio  power  output  -f-  power  intake 
is  called  the  efficiency  of  a  transformer. 

Table  VII,  shows  the  efficiencies,  losses,  and  regulation 
of  a  recent  series  of  combined  core-  and  shell-type  transformers 
designed  and  manufactured  by  a  large  American  company.  These 
transformers  are  designed  for  a  frequency  of  60  cycles  per  second  and 
primary  voltages  of  1,100  or  2,200  volts,  according  to  whether  the  two 


264 


ALTERNATING-CURRENT  MACHINERY 


"'as*} 


T 

95.X% 


$IOM 


!jtOM 


halves  of  the  primary  coil  are  connected  in  parallel  or  in  series.    The 
secondary  voltages  are  110  or  220  volts. 

Fig.  235  shows  graphically  the  relation  between  the  efficiency 
and  the  output  for  a  7.5-kilowatt,  core-type  transformer  designed 
for  primary  voltages  of  1,040  and  2,080,  and  secondary  voltages  of 
104  and  208,  and  a  frequency  of  60  cycles  per  second.  The  core 
loss  is  86.5  watts  and  the  total  copper  loss  at  full  load  is  117.8  watts. 
The  high  efficiency  throughout  a  wide  range  of  load  is  worthy  of  note 
and  is  typical  of  all  well-designed  transformers. 

Fig.  236  shows  graphically  the  various  losses  and  the  efficiency 
of  a  Westinghouse  air-blast  transformer  rated  at  550  kilowatts  used 

to  step  up  from  500  volts 
to  10,500  volts  at  a  fre- 
quency of  25  cycles  per 
second  (3,000  alternations 
per  minute). 

In  Fig.  236,  the  curve 
representing  the  iron  loss 
is  plotted  as  a  horizontal 
straight  line  because  the 
iron  loss  for  a  given  trans- 
former is  practically  con- 
stant for  all  loads.  The 
curve  representing  the 
copper  loss  is  a  parabola. 
The  efficiency  of  a  given  transformer  is  a  maximum  at  that  load  for 
which  the  iron  loss  is  equal  to  the  copper  loss.  This  load  is  evidently 
the  abscissa  of  the  point  at  which  the  iron-loss  line  intersects  the  cop- 
per-loss curve.  As  seen  in  Fig.  236,  the  maximum  efficiency  occurs  at 
about  101  per  cent  of  full  load;  at  any  other  load  (for  the  given 
transformer)  the  efficiency  will  be  less  than  at  101  per  cent  of  full  load. 
The  transformer  output  (non-inductive  receiving  circuit)  is 
E"I".  The  internal  loss  is  Wh + We  +  Wc,  so  that  the  intake  is  E"I" 
+  Wh+We+We,  and  the  efficiency  is 


O      tO      XO     3O 


K/LOWATTJ     OU7fUT 
X  J  •*  5  e 

?O     5O     GO     70     SO      90    IOO  //<?    HO   t3O 
PFR  CENT   FULL  LOAD 


Fig.  235.     Graphic  Relation  between  Efficiency  and 
Output  for  a  Coil  Type  Transformer 


efficiency  =          , 


(38) 


A  complete  calculation  of  efficiency  is  worked  out  on  page  314. 


ALTERNATING-CURRENT  MACHINERY 


265 


All-day  Efficiency.  Usually  a  transformer  is  connected  to  the 
mains  continuously,  and  current  is  taken  from  the  secondary  for  a 
few  hours  only,  each  day.  In  this  case  the  iron  loss  is  incessant  and 
the  copper  loss  is  intermittent.  The  total  work  given  to  the  trans- 
former during  the  day  may  greatly  exceed  the  total  work  given  out 
by  it,  especially  if  the  continuous  iron  losses  are  not  reduced  to  as 


r 

WESTINGHOUSE  A1R-BLAS 
550K.W.  I0.500VOLT3,  3, 

T    TRANSFORMER 
000  ALTERNATIONS 

X- 

/ 

| 

TABLE  OF  EFFICIENCIES 

l/s    Load     -98.2% 
\VA       "         =98.29 
Full     "         =96.32 
&       "         -98.34 
Yi       "        =97.66 
'/4       ••        =96.43 

X 

/ 

S 

^ 

^< 

X 

^ 

r_--^- 

-^ 

jsT' 

' 

/ 

—  === 



-     —  • 

,  - 

r  i  i 

RON  LOSS) 

—  - 

^ 

^ 

. 

t 

B 

)^ 



•       — 

-    — 

_    -~ 

.  

^-*-*> 

<. 

_ 

_ 

.0  3 

?; 

e   ^ 


50 


75 


PER    CENT  rLOAI> 

Fig.  236.     EflSciency  Curves  for  a  Westinghouse  Air-Blast  Transformer 


low  a  value  as  possible.  The  ratio  total  energy  given  out  by  the  trans- 
former -T-  total  energy  received  by  the  transformer  during  the  day  is- 
called  the  "all-day  efficiency"  of  the  transformer.  In  other  words 

total  watt-hours  output 

it  is  the  ratio  -  — —    — -r±  —-  during  the  day.  The  all-day 

total  watt-hours  input 

efficiency  is  given  by  the  formula 


all-day  efficiency  = 


E'T'Xt 


E'T'X  t+  (Wh+We)  X24  +WCX  t 


(39) 


in  which  t  is  the  number  of  hours  during  the  day  of  24  hours  that 
the  transformer  is  loaded,  and  1"  is  the  average  current  delivered 
by  the  secondary  while  the  transformer  is  loaded.  The  other  symbols 
have  the  same  significance  as  on  pages  262-264. 


266          ALTERNATING-CURRENT  MACHINERY 

Since  the  iron  loss  of  a  given  transformer  is  continuous  as  long 
as  the  transformer  is  connected  to  the  primary  supply  mains,  it 
follows  that  to  obtain  a  high  all-day  efficiency,  it  is  necessary  to  use 
a  transformer  whose  iron  loss  is  as  small  as  possible.  In  general,  if 
a  transformer  is  to  be  operated  at  light  loads  the  greater  part  of  the 
day,  as  is  the  case  in  electric  lighting  service,  it  is  much  more  econom- 
ical to  use  a  transformer  designed  for  a  small  iron  loss  than  for  a  small 
full-load  copper  loss.  In  very  small  transformers,  the  iron  and  the 
copper  losses  are  made  about  equal,  but  for  outputs  of  about  5 
kilowatts  and  upwards,  the  iron  loss  is  often  made  only  about  one-half 
as  great  as  the  full-load  copper  loss.  This  is  illustrated  by  the  data  in 
the  second  and  third  columns  in  Table  VII. 

On  the  other  hand,  in  cases  where  transformers  are  to  be  used 
to  supply  a  load  of  induction  motors,  and  the  conditions  are  such 
that  the  transformers  are  operated  at  or  near  full  load  during  most 
of  the  day,  it  is  more  economical  to  use  transformers  designed  to  give 
equal  iron  and  copper  losses  at  full  load. 

In  general,  a  given  transformer  works  at  the  maximum  efficiency 
when  it  is  operating  at  a  load  such  that  its  iron  loss  is  equal  to  its 
total  copper  loss. 

Transformer  Regulation.  The  secondary  terminal  voltage  of 
a  transformer  falls  off  in  value  with  increasing  load,  and  rises  with 
decreasing  load.  The  rise  of  secondary  terminal  voltage  from  rated 
non-inductive  load  to  no  load  (at  constant  primary  applied  terminal 
voltage),  expressed  in  per  cent  of  the  secondary  terminal  voltage 
at  rated  load,  is  called  the  regulation  of  a  transformer. 

Example.  A  certain  5-kw.  transformer  gave  a  secondary  terminal 
voltage  of  200  volts  at  rated  non-inductive  load;  when  the  load  was  reduced 
to  zero,  the  voltage  rose  to  203  volts. 

In  accordance  with  the  above  definition,  the  regulation  of  this  transfer- 

meiiS  (203-200) 

-  X  100  =  1.5  per  cent 
^300 

The  regulation  of  the  average  distributing  transformers  on  the 
market  at  present  varies  from  about  1  per  cent  to  3.5  per  cent,  and 
when  operated  on  power  factors  as  low  as  60  per  cent,  the  regula- 
tion in  some  cases  is  as  high  as  4  per  cent.  In  incandescent  lighting 
it  is  especially  important  to  have  transformers  with  a  low  regulation, 
otherwise  the  voltage  at  the  lamps  will  fluctuate  excessively  as  the 


ALTERNATING-CURRENT  MACHINERY  267 

load  changes.  This  means  unsatisfactory  illumination  for  the  con- 
sumer, and  more  frequent  lamp  renewals  for  the  lighting  company. 
Thus,  for  lighting  work  it  is  important  to  specify  a  low  regulation. 

The  regulation  of  a  transformer  is  lower  when  used  for  supply- 
ing a  non-inductive  load  (such  as  incandescent  lamps),  than  when 
supplying  an  inductive  load  (such  as  induction  motors).  For  a  given 
kind  of  load  the  regulation  of  large  transformers  is  lower  than  for 
small  sizes.  These  matters  are  clearly  brought  out  by  a  careful 
study  of  the  values  of  regulation  given  in  Table  VII.  The  method  of 
calculating  regulation  is  explained  on  page  313. 

Practical  and  Ultimate  Limits  of  Output.  When  the  secondary 
current  of  a  transformer  is  increased,  the  secondary  electromotive 
force  drops  off,  and  the  power  output  increases  with  the  current, 
reaching  a  maximum  as  in  the  case  of  the  alternator.  This  maximum 
power  output  is  the  ultimate  limit  of  output  of  the  transformer. 
Practically,  the  output  of  a  transformer  is  limited  to  a  much  smaller 
value  than  this  maximum  output,  (a)  because  of  the  necessity  of 
cool  running;  (b)  because  in  most  cases  it  is  necessary  that  the 
secondary  electromotive  force  be  nearly  constant;  and  (c)  because 
the  efficiency  of  a  transformer  is  low  at  excessive  outputs. 

Small  transformers  have  relatively  large  radiating  surfaces; 
and  in  such  transformers  the  requirements  of  a  small  (good)  regula- 
tion, as  a  rule,  determine  the  allowable  output. 

Large  transformers,  on  the  other  hand,  have  relatively  small 
radiating  surfaces,  and  their  allowable  output  is  limited  by  the 
permissible  rise  in  temperature.  Some  transformers  ranging  in  rated 
output  from  100  to  1,250  kilowatts  are  provided  with  air  passages 
through  which  air  is  made  to  circulate  by  a  fan.  Transformers  which 
are  not  cooled  by  an  air  blast  up  to  about  500  kilowatts  capacity  are 
submerged  in  oil,  which,  by  convection,  carries  heat  from  the  trans- 
former to  its  enclosing  case,  where  it  is  radiated.  Very  large  trans- 
formers are  not  only  submerged  in  oil,  but  are  also  water-cooled. 

Rating  of  Transformers.  A  transformer  is  rated  according 
to  the  power  it  can  deliver  continuously  to  a  non-inductive  receiv- 
ing circuit  without  undue  heating;  and  the  ratio  of  transformation, 
together  with  a  specification  of  the  frequency  and  effective  value  of 
the  primary  electromotive  force  to  which  the  transformer  is  adapted, 
are  also  given  by  the  manufacturer. 


268  ALTERNATING-CURRENT  MACHINERY 

In  order  to  secure  uniformity  in  the  rating  of  transformers  by 
manufacturers,  the  American  Institute  of  Electrical  Engineers  rec- 
ommends the  following  rules  concerning  the  allowable  rise  of  tem- 
perature: 

(1)  In  transformers  for  continuous  service,  the  temperature  rise  should 
not  exceed  50°C.  as  measured  by  change  of  electrical  resistance  of  primary 
and  secondary  coils  and  by  thermometer  in  other  parts. 

(2)  In  transformers  intended  for  intermittent  service,  or  not  operating 
continuously  at  full  load,  but  continuously  in  circuit,  as  in  the  ordinary  case 
of  lighting  transformers,  the  temperature  elevation  above  the  surrounding 
air  temperature  should  not  exceed  50°C.  as  measured  by  resistance  in  electric 
circuits,  and  by  thermometer  in  other  parts,  after  the  period  corresponding 
to  the  term  of  full  load.    In  this  instance,  the  test  load  should  not  be  applied 
until  the  transformer  has  been  in  circuit  for  a  sufficient  time  to  attain  the 
temperature  elevation  due  to  core  loss.      With  transformers  for  commercial 
lighting,  the  duration  of  the  full  load  test  may  be  taken  as  three  hours,  unless 
otherwise  specified. 

The  over-load  rating  of  a  transformer  is  recommended  to  be 
25  per  cent  for  two  hours;  and  it  is  based  upon  an  allowable  increase 
of  temperature  of  15°  C.  above  that  specified  for  full  load,  as  given 
above.  This  extra  rise  of  temperature  is  to  be  measured  after  the 
transformer  has  been  operating  for  two  hours  on  25  per  cent  over- 
load, the  transformer  having  previously  acquired  the  temperature 
corresponding  to  full-load  continuous  operation. 

When,  however,  transformers  are  to  be  used  with  other  apparatus 
for  which  an  over-load  capacity  in  excess  of  25  per  cent  is  guaranteed, 
the  same  guarantee  should  apply  to  the  transformers. 

Abnormal  Conditions  of  Operation.  Transformers  are  fre- 
quently used  under  conditions  departing  more  or  less  widely  from 
the  conditions  specified  by  the  manufacturer  in  regard  to  values  of 
primary  and  secondary  voltages,  frequency,  and  current  output. 
Thus,  if  a  transformer  is  used  with  a  primary  applied  voltage  in 
excess  of  the  rated  value  (frequency  being  unchanged),  the  core 
flux  will  be  increased  according  to  equation  (34)  and  the  core  losses 
will  be  increased  according  to  equations  (35)  and  (36). 

If,  on  the  other  hand,  a  transformer  is  used  with  a  frequency 
less  than  the  rated  frequency  (the  voltage  applied  to  the  primary 
being  unchanged),  the  core  flux  will  be  increased  according  to  equa- 
tion (34),  and  the  hysteresis  loss  in  the  core  will  be  increased  according 
to  equation  (35);  whereas,  the  eddy-current  loss  is  un  changed. 


ALTERNATING-CURRENT  MACHINERY  269 

The  increase  of  core  loss  due  to  increase  of  primary  applied 
voltage,  or  to  decrease  of  frequency,  or  to  both,  may  be  compen- 
sated for  by  reducing  the  allowable  current  output,  and  thereby 
reducing  the  copper  loss,  in  order  that  the  total  heating  of  the  trans- 
former may  not  exceed  the  normal  amount. 

Examples.  A  given  transformer  is  rated  at  5  kilowatts,  and  is  designed 
to  take  current  from  1,100-volt  mains  at  a  frequency  of  60  cycles  per  second. 
Under  these  conditions  hysteresis  loss  Wh,  eddy  current  loss  We,  and  copper 
loss  Wc,  will  be  called  normal. 

(a)  The  transformer  is  loaded  so  that  the  output  is  6  kilowatts  at  the 
rated  electromotive  force  and  frequency.    Find  Wc  in  terms  of  normal. 

(b)  The  transformer  is  used  at  rated  electromotive   force,    but    at  a 
frequency  of  75  cycles  per  second.    Find  Wh  and  We  each  in  terms  of  normal. 

(c)  The  transformer  is  used  at  rated  frequency,   but  with  primary 
electromotive  force  of  1,500  volts.    Find  Wh  and  We  each  in  terms  of  normal. 

(d)  The  transformer  is  used  on  primary  electromotive  force  of  1,500 
volts.     Find  /  for  which  W^  is  normal. 

(e)  With  primary  electromotive  force  of  1,500  volts,  what  load  would 
give  normal  Wct 

Solutions,  (a)  Increasing  the  output  in  the  ratio  5  to  6,  increases  both 
primary  current  and  secondary  current  in  the  same  ratio  and,  therefore,  in- 
creases I'2Rf  and  I"2R"  in  the  ratio  of  52  to  62.  Therefore,  the  total  copper 

Q  A 

loss  becomes  —  =  1.44  times  the  normal  copper  loss. 
2o 

(b)  Increasing^  the  frequency  in  the  ratio  60  to  75  decreases  the  flux- 
density  B  in  the  same  ratio,  namely,  75  to  60.     Hence,  the  hysteresis  loss  per 
cycle  is  decreased  in  the  ratio  751'6  to  60U>,    and  the  total  hysteresis  loss  is 

75       60  16        /60y6 

—  X  —  1<6  or   (-—  J^  or  o.87.    Therefore,  the  total  hysteresis  loss  at  the  in- 

creased frequency  is  0.87  times  the  normal  hysteresis  loss. 

From  equation  (36),  the  eddy  current  loss  is  proportional  to  /2  B2. 
In  the  case  under  consideration,  /  is  increased  and  B  is  decreased  in  the  same 
ratio,  so  that  the  product  f^B?  remains  unchanged.  Hence,  the  eddy-current 
loss  in  a  given  transformer  is  independent  of  the  frequency  with  given  primary 
applied  voltage. 

(c)  Increasing  the  primary  voltage  in  the  ratio  11  to  15  increases  the 
flux-density  B  in  the  same  ratio.    Therefore,  the  hysteresis  loss  is  increased 

/ISX1'6 
in  the  ratio  (  —  )     =1.64;  and  the  eddy  current  loss  is  increased  in  the  ratio 

/  15  \2 

f  —  )    =  1.86.     That  is,  the  hysteresis  loss  becomes  1.64  times  its  normal 

value;  and  the  eddy  current  loss  becomes  1.86  times  its  normal  value. 

(d)  The  hysteresis  loss  is  proportional  to/B1'6;  and  B  is  proportional 

jftr  x  jjir    *   1.6  ET/1.6 

to  —  -  ;  therefore,  the  hysteresis  loss  is  proportional  to  /  X  (  —  )     or  to     0  v  • 


This  ratio  —  -^  must  have  the  same  value  under  the  normal  conditions  as  under 


270 


ALTERNATING-CURRENT  MACHINERY 


the  new  conditions,  if  the  hysteresis  loss  is  to  be  the  same.     That  is 

(1,100)V6       (1,500)K6 


(60)( 


or 


(1,100)  •  X  z°'6  =(60)°-6  X  (l,500r6 
or  taking  logarithms  of  both  sides,  we  have 

(l.GXlog  1, 100)+ (0.6  X  log  z)  =  (0.6Xlog  60)  +  (1.6  X  log  1,500);  or, 
(0.6Xlog  x)  =  (0.6  X  log  60)  +  (1.6  X  log  1,500)  -  (1.6  X  log  1,100);  from  which 
we  find  x  equal  to  137  cycles  per  second. 

(e)  The  copper  loss  Wc  has  normal  value  when  the  primary  and  sec- 
ondary currents  have  normal  value.  Therefore,  when  the  primary  applied 
voltage  is  increased  in  the  ratio  11  to  15,  the  output  increases  in  the  same  ratio, 

the  current  and  Wc  being  normal.    Therefore,  the  output  is  — —   X    5  kw.,  or 
6.82  kilowatts,  to  give  normal  TFcwith  a  primary  applied  voltage  of  1,500  volts. 

COMMERCIAL  TYPES  OF  TRANSFORMERS 

Transformers  may  be  classified,  according  to  the  relative  dis- 
position of  the  iron  and  copper,  into  core-type  transformers  and  shell- 
type  transformers.  The  distinc- 
tion between  these  two  types  of 
transformer  maybe  understood 
by  referring  to  Figs.  237  and  267. 
In  the  core  type,  Fig.  237,  it  will 
be  seen  that  the  iron  core  is 
almost  entirely  surrounded  by 
the  copper  windings,  while  in  the 
shell  type,  Fig.  267,  the  coils  are 
almost  entirely  surrounded  by 
the  sheet-iron  laminations,  form- 
ing a  shell. 

There  has  been  much  discus- 
sion as  to  the  relative  merits  of 
the  shell  and  core  types,  some  manufacturers  going  so  far  as  to  claim 
exclusive  advantages  for  one  or  the  other.  The  fact  is  that  no  gen- 
eral conclusion  can  be  drawn  as  to  which  type  is  better,  for  each 
possesses  inherent  characteristics  which  specially  adapt  it  to  certain 
conditions.  A  brief  comparison  of  these  characteristics  will  aid  in 
determining  which  type  to  use  for  specified  conditions  of  service,  size, 
voltage,  and  the  like. 


Fig.  237.    Core-Type  of  Transformer 
Assembled  with  Case  Removed 


ALTERNATING-CURRENT  MACHINERY 


271 


Fig.  238.     Standard 

Form  of  Laminated 

Transformer  Core 


The  core  type  has  relatively  a  lighter  core  of  smaller  sectional 
area  but  a  greater  length  of  magnetic  circuit,  while  the  copper  is 
relatively  heavier,  containing  more  turns,  although  of  shorter  mean 
length.  The  core  type  is  more  easily  wound  as  cyl- 
indrical formed  coils  may  be  used,  and  the  coils  are 
more  accessible  and  expose  more  surface  to  radia- 
tion. The  core  type,  with  its  relatively  large  wind- 
ing space,  is  better  adapted  for  high  voltages  which 
require  many  turns  and  large  space  for  insulation, 
smaller  currents  and,  therefore,  small  wires,  and 
higher  frequencies  with  low  magnetic  flux  densities. 

The  shell  type,  on  the  other  hand,  is  particu- 
larly suited  for  transformers  of  moderate  voltage, 
requiring  few  turns  and  little  insulation,  large  cur- 
rents, and  low  frequency  with  corresponding  mag- 
netic flux. 

The  net  result  is  that  manufacturers  generally  adopt  the  core 
type  for  transformers  of  small  capacity  and  high  voltage,  and  the 
shell  type  for  large  transformers,  even  up  to  150,000  volts. 

Substantial  advance- 
ment in  transformer  de- 
sign has  been  made  in  the 
past  few  years,  notably 
in  the  combination  of  the 
advantages  of  both  the 
core  and  the  shell  types  in 
one  transformer,  in  the 
use  of  silicon-steel  for  the 
laminations,  thus  greatly 
reducing  the  core  loss,  in 
the  use  of  better  insulat- 
ing materials  and  meth- 
ods, and  in  more  effective  methods  of  cooling.  With  these  improved 
materials  and  methods  it  is  even  feasible  to  build  reliable  transformers 
with  outputs  up  to  14,000  kilo  volt  amperes,  and  for  voltages  up  to 
150,000,  or  higher. 

Core  Type.  Fig.  237  shows  a  complete  core-type  transformer 
without  its  case;  Fig.  238  shows  the  complete  core  built  up  of  thin 


Fig.  239.     Core  with  Upper  Yoke  Removed,  Showing  One 
Coil  in  Position 


272 


ALTERNATING-CURRENT  MACHINERY 


sheet  steel  strips,  or  stampings.     The  two  upright  portions  of  the 
core  upon  which  the  wdre  coils  are  placed,  as  shown  in  Figs.  237, 239, 


Fig.  240.     Both  Transformer  Coils  in  Position,  Showing 
Laminated  Strips  Which  Form  the  Upper  Yoke  of  Core 


and  240,  are  called  the  limbs  of  the  core.    The  short  horizontal  parts 

of  the  core  that  do  not  have  windings  of  wire  upon  them  are  called 

yokes.     These  yokes  serve  to  complete  the  magnetic  circuit,  and 

they  are  made  just  long  enough  to  give 

room  for  the  coils,  as  shown  in  Fig.  241. 

Fig.  239  shows  the  core  with  its  upper 

connecting  yoke  removed  to  permit  of 

the  slipping  of  the  coils  into  position 

on  the  core;  the  left-hand  limb  of  the 

core  is  shown  wrapped  with  a   thick 

layer  of   insulating  material  in  order 

to  prevent  electrical  contact  between 

the  wire  of  the  coil  and  the  iron  of  the 

COre.       Fig.    240     shoWS     the     Coils     in         Fig.  241.     Section  Through  Core- 

.  Type  of  Transformer 

place,  and  a  pile  of  loose   sheet-iron 

stampings   which   are  used   to   form    the  upper   connecting  yoke. 

Fig.  241  is  a  sectional  view  of  the  completed  core  and  coils; 

and  Fig.  242  is  a  sectional  view  of  the  complete  transformer  enclosed 


ALTERNATING-CURRENT  MACHINERY 


273 


in  a  cast-iron  case;  which  is  usually  filled  with  oil.  Half  of  the  primary 
coil  and  half  of  the  secondary  coil  also  is  placed  upon  each  limb  of  the 
core,  as  shown  by  the  sec- 
tional view,  Fig.  241.  The 
terminals  of  each  half  of  the 
primary  (fine-wire)  coil  are 
connected  to  binding  screws 
on  a  porcelain  connection 
board  mounted  on  top  of 
the  transformers  inside  the 
case,  as  shown  at  the  right 
in  Fig,  242.  The  terminals 
of  each  half  of  the  secondary 
(coarse-wire)  coil  are  passed 
through  porcelain  bushings 
to  the  outside  of  the  case,  as 
shown  at  the  right  in  Fig. 
242.  The  half  coils  of  the 
primary  may  be  connected  in 
parallel  or  in  series  according 
to  the  value  of  the  voltage  applied  to  the  primary;  and  the  half  coils 
of  the  secondary  may  be  connected  in  parallel  or  in  series  according 
to  the  desired  value  of  the  secondary  voltage,  as  explained  on  page  244. 


Fig.  242.     Section  Through  Commercial 
Core-Type  of  Transformer 


Primaries  in  Series 


Primaries  in  Parallel 


Secondaries 
inSeriesorThree-wire 


Secondarfes 
in  Parallel 


Fig.  243. 


Plan  of  Transformer  Showing  Strap  Connections  for  Arranging 
Primaries  in  Series  or  Parallel 


Fig.  243  is  a  top  view  of  the  transformer  with  the  cover  of  its 
case  removed  showing  the  terminals  of  the  secondary  coils,  and  the 


274          ALTERNATING-CURRENT  MACHINERY 

porcelain  connection-board  to  which  the  terminals  of  the  primary 
coil  are  connected.  The  change  from  series  to  parallel  connection 
of  the  halves  of  the  primary  coils,  is  effected  by  means  of  the  copper 
connecting  straps  S  S. 

The  style  of  core-type  transformer  illustrated  in  Figs.  237  to 
242  is  adopted  by  several  manufacturers  for  transformers  of  small 
output.  The  style  of  core-type  transformer  adopted  by  the  Gen- 
eral Electric  Company  for  larger  outputs  up  to  350  kilowatts,  is 
illustrated  in  Figs.  244  to  248,  which  show  a  high  voltage  core-type 
transformer  designed  for  60  cycles.  These 
transformers  are  submerged  in  tanks  of  oil, 
one  of  which  is  shown  in  Fig.  248,  and  which 
are  made  with  a  cast-iron  base  and  sides  of 
heavy  steel  with  deep  corrugations  to  fa- 
cilitate cooling  of  the  tank  by  radiation. 
Fig.  246  shows  the  arrangement  of  the 
coils  on  the  core;  it  also  shows  the  pas- 
sages between  the  core  and  the  inside  coil, 
and  between  the  two  coils,  for  the  circula- 
tion of  the  oil. 

Oil  is  a  better  heat-conducting  medium 
than  air;  it  carries  heat  from  a  transformer 
to  the  containing  case  much  better  than 
air,  so  that  a  transformer  in  oil  will  show  a 
much  lower  temperature.  The  use  of  oil, 

Fig.  244.   General  Electric  Core-  .  . 

Type  Transformer  Partly      moreover,  preserves  the  insulation,  keeping 

Assembled  . 

it  soft  and  pliable,  and  preventing  oxidation 

by  air;  consequently,  its  use  is  advantageous  in  producing  proper 
conditions  to  maintain  uniform  core  loss  and  a  superior  insula- 
tion. Furthermore,  oil  is  itself  a  very  good  insulator  having 
the  valuable  property  common  to  all  liquid  insulators,  that 
it  is  not  .permanently  damaged  by  a  puncture  caused  by  lightning; 
for  example,  in  this  case  the  resistance  of  the  oil  is  only  momen- 
tarily broken  down,  the  oil  immediately  flowing  into  the  break  and 
sealing  the  insulation. 

Another  variety  of  the  core  type  is  that  adopted  by  the  Crocker- 
Wheeler  Company  in  their  "remek-type"  transformer.  The  core 
punchings  are  shown  in  Fig.  249.  Each  lamination  consists  of  a 


ALTERNATING-CURRENT  MACHINERY  275 

core  and  yoke  punching,  which  are  punched  simultaneously  from 
the  whole  sheet  by  a  compound  die.  In  assembling  the  transformer, 
care  is  taken  to  get  the  core  and  yoke  punchings  back  into  the  same 


Fig.  245.     General  Electric  Core-Type  Transformer 
with  Tank  Removed 


relative  position  as  before  punching,  in  order  to  secure  the  lowest 
possible  magnetic  reluctance  of  the  joints,  and  hence  a  small  mag- 
netizing current.  The  laminations  when  assembled  are  held  to- 


276 


ALTERNATING-CURRENT  MACHINERY 


gether  by  iron  end  plates  bolted  together  and  the  core  portions  are 
securely  clamped  together  and  to  the  yokes,  as  shown  in  Fig.  250. 
The  core,  as  seen,  constitutes  a  double  mag- 
netic circuit,  the  magnetic  flux  due  to  the 
windings  on  the  central  core  dividing  on  pass- 
ing into  the  yokes,  half  on  each  side.  The 
yoke  laminations  are  made  wider  than  one- 
half  the  width  of  the  central  core,  in  order  to 
reduce  the  flux  density  in  them,  thereby  re- 
ducing the  core  loss  and  the  magnetizing  cur- 
rent. 

The  windings  consist  of  former-wound 
coils  which  surround  the  core  as  illustrated  in 
Fig.  25 1 .  The  high  voltage  winding  is  placed 
between  the  two  halves  of  the  low  voltage 
winding  in  order  to  reduce  magnetic  leakage 
and  thus  improve  regulation. 

Vertical  ducts  are  provided  between  the  windings,  and  between 
the  core  and  windings,  as  may  be  seen  in  Fig.  251.  This  arrange- 
ment is  to  insure  a  free  circulation  of  the  oil  in  which  the  transformer 
is  immersed  and  facilitates  dissipation  of  the  heat  by  radiation  and 
convection. 

The  assembled  transformer  core  and  coils  ready  to  be  placed 
in  the  oil-filled  tank  are  shown  in  Fig.  252.  The  high  voltage  termi- 


Fig.  246.     Coils  and  Core  of 
General  Electric  Trans- 
former Showing 
Oil  Ducts 


Fig.  247.    Specimen  Coils  for  General  Electric  Core-Type  Transformer 

nals  are  shown  connected  to  a  terminal  board  in  front,  and  the 
four  secondary  terminals,  two  for  each  coil,  are  shown  in  the  rear. 


ALTERNATING-CURRENT  MACHINERY 


277 


These  "remek-type"  transformers  are  wound  for  a  primary 
voltage  of  2,200,  secondary  voltages  of  220  and  110,  and  are  built 
in  sixteen  sizes  ranging  from  0.6  to  50  kilowatts. 

Fig.    253   shows   a    10-kw.    combination   core-   and   shell-type 


Fig.  248.     General  Electric  Core-Type  Transformer 
Completely  Assembled  Showing  Deep  Corruga- 
tions to  Facilitate  Cooling 

transrormer  removed  from  its  tank.  It  is  called  the,  distributed 
core  type  (type  H  form  K)  by  its  makers,  the  General  Electric  Com- 
pany, although  it  resembles  the  shell  rather  than  the  core  type. 


278  ALTERNATING-CURRENT  MACHINERY 

The  primary  and  the  secondary  coils  are  wound  on  formers  of 
cylindrical  shape,  and  placed  on  the  center  limb  only.  They  are 
separated  from  each  other  and  from  the  core  by  spacing  blocks, 
thus  forming  ducts  and  passages  for  free  circulation  of  the  oil  with 
which  the  containing  tank  is  filled.  Fig.  254  shows  clearly  the 
disposition  of  the  coils,  mica  insulation,  and  oil  ducts,  with  respect 
to  the  central  limb. 

The  core,  as  shown  in  Fig.  255,  contains  four  magnetic  circuits  in 
parallel,  each  circuit  consisting  of  a  separate  core  similar  in  general 
outline  to  that  used  in  the  simple  core  type.  One  limb  of  each 
magnetic  circuit  is  built  up  of  two  different  widths  of  punchings, 
forming  a  cross-section  such  that  when  the  four  circuits  are  assem- 


Fig.  249.     Core  Punchings  for  Crocker-Wheeler  "Remek-Type"  Transformer 

bled  together  they  interlock  to  form  a  common  central  limb  upon 
which  the  primary  and  the  secondary  coils  are  placed. 

The  four  remaining  legs  consist  of  punchings  of  equal  width. 
These  occupy  a  position  surrounding  the  coil  at  equal  distances 
from  the  center,  on  the  four  sides,  forming  a  channel  between  each 
leg  and  coil,  thereby  presenting  large  surfaces  to  the  oil  and  allow- 
ing it  free  access  to  all  parts  of  the  winding. 

The  punchings  of  eadi  size  of  transformer  are  all  of  the  same 
length,  assembled  alternately,  and  forming  two  lap  joints  equally 
distributed  in  the  four  corners  of  the  core,  thereby  giving  a  mag- 
netic circuit  of  very  low  reluctance. 

This  type  of  construction  combines  the  best  features  of  both 
shell  and  core  types,  namely  a  short  mean  length  of  turn  in  the 
windings,  and  a  short  length  of  magnetic  circuit  in  the  core. 


ALTERNATING-CURRENT  MACHINERY 


279 


Fig.  250.     Assembled  Core  Show- 
ing Method  of  Clamping 


Fig.  251.     Method  of  Assembling 
Coils  Showing  Oil  Ducts 


Fig.  252.     Assembled  "Remek"  Core  and  Coils 


280  ALTERNATING-CURRENT  MACHINERY 


Fig.  253.     General  Electric  Combination  Core  and  Shell  Type  Transformer 


r 


'M/CA  SH/ELO 

M/CA  SM&-0—-A 


SECOMfijffy 


L 


Fig.  254.     Details  of  Core  and  Coils  Showing  Oil  Ducts 


ALTERNATING-CURRENT  MACHINERY  281 


MCA  Stt/fLDS 


•SECONDARY 
Pft/flARY 


0/2. 
DUCT 


Fig.  255.     Plan  of  Assembled  Core  and  Coils 
Showing  Four  Magnetic  Circuits  in  Parallel 


Fig.  256.     Westinghouse  Combination  Shell  and  Core  Type 


Fig.  257.     Assembled  Coils  for  Westinghouse  Transformer 


282 


ALTERNATING-CURRENT  MACHINERY 


Another  example  of  the  combined  shell-  and  core-type  of  trans- 
former is  the  distributing  transformer  (types  S  and  SA)  of  the  West- 


Fig.  258.     Section  through  Westing- 
house  "Type  S"  Transformer 


Fig.  259.     Part  Section  through 
Westinghouse  Transformer 


Fig.  260.     Pancake  Coil  for 
Shell-Type  Transformer 


Fig.  261.     Pancake  Coil 
We 


found  with  Tape 

inghouse  Electric  Company.  These  transformers  are  almost  iden- 
tical in  form  and  design  with  the  "type  H  form  K"  transformers 
just  described. 


ALTERNATING-CURRENT  MACHINERY 


283 


Fig.  256  shows  the  core  and  coils  and  terminals  of  the  "type 
S"  transformers  removed  from  the  case,  and  a  general  view  of  the 
case.     Fig.  257   shows   the   appearance   of  the 
assembled   coils    and   leads   of    10-  and   15-kw. 
transformers,  with   the  ventilating   ducts.     The 
figure  shows  the  high  voltage  winding  mounted 
concentrically  between   two   low   voltage   wind- 
ings, which  arrangement  reduces  magnetic  leak- 
age and  thus  improves  "regulation." 

As  illustrated  in  Fig.  256,  separate  high 
and  low  voltage  porcelain  terminal  blocks  are 
mounted  upon  extensions  of  the  upper  end 
frame,  and  are  thus  kept  well  apart  to  prevent 
mistakes  in  making  electrical  connection  between 
the  high  and  low  voltage  coils. 

Fig.  258  is  a  section  through  a  J-kw.  "type 

,     Fig.  262.     Pancake  Coil 

S     transformer,  and  shows   the   distribution   of     showing  Use  of  insu- 

lated  Flat  Strips 

the   windings   in   layers,    the  high  voltage  coils 

being  placed  between  two  sections  of  low  voltage  coils.     Fig.  259 

is  a  section  through  a  50-kw.  transformer  and  shows  the  large  oil 

ducts  arranged  between  sections   of   the  high 

voltage    winding.      The    angle    irons  used  to 

clamp  the  laminations  together  are  shown  at 

the  top  and  bottom.     These  distributing  "type 

S"  transformers  are  built  in  sixteen  sizes  from  \ 

to  50  kilowatts,  and  for  the  standard  primary 

voltages   of   2,200    and   1,100,  and   secondary 

voltages  of  220  and  110.     Standard  frequencies 

are  25,  40,  and  60  cycles. 

Shell  Type.  In  shell-type  transformers 
the  coils,  both  primary  and  secondary,  are  usu- 
ally wound  in  pancake  form  on  formers,  as 
shown  in  Figs.  260  to  263.  The  coils  in  small 
transformers  are  wound  with  round  wire,  but  in 
the  larger  sizes  flat  rectangular  copper  strip  is 
used  with  one  turn  per  layer,  in  many  layers,  Figs. 
260  and  262.  The  insulation  between  turns  consists  of  paper,  mica, 
or  varnished  cambric,  or  all  three  together,  according  to  the  voltage 


Fig.  263.     Method  of 

Binding  up  Pancake 

Strip  Coils 


284  ALTERNATING-CURRENT  MACHINERY 

and  other  conditions.  The  thin  pancake  coils  are  then  treated 
with  an  insulating  compound  and  wound  with  a  number  of  layers 
of  tape  according  to  the  voltage  for  which  they  are  designed,  each 
layer  being  given  several  coats  of  insulating  varnish  baked  on  in 
ovens.  The  coils  are  then  assembled  into  groups  of  two  or  more 
sections,  Fig.  263,  and  the  groups  into  complete  windings,  the  pri- 
mary and  the  secondary  being  intermixed  or  sandwiched  in  order  to 


Fig.  264.     Partially  Assembled  Air  Blast  Shell-Type  of 
Transformer 

reduce   magnetic   leakage.     Between   the   various   groups   suitable 
insulating  barriers  are  interposed. 

In  transformers  for  high  voltages  used  for  transmission  of 
power,  the  insulation  of  a  considerable  length  of  the  conductor 
nearest  the  terminal  leads  is  heavily  reinforced.  This  is  a  very 
important  precaution,  as  the  extra  dielectric  strength  of  the  insu- 
lation of  these  end  turns  is  a  safeguard  against  break-downs  which 
might  otherwise  occur  due  to  the  excessive  voltages  from  light- 
ning discharges  or  other  surges  to  which  transmission  lines  are 
unfortunately  subjected, 


ALTERNATING-CURRENT  MACHINERY 


285 


The  various  groups  of  windings  are  encased  in  a  box-like  struc- 
ture which,  while  serving  as  an  electrical  and  a  mechanical  protection, 
is  so  arranged  that  it  does  not 
obstruct  the  air  or  oil  ducts 
which  are  provided  between 
the  various  coil  sections.  The 
windings  are  then  set  up  verti- 
cally in  the  bottom  frame  and 
the  magnetic  circuit  is  built  up 
piece  by  piece  around  them  in 
the  form  of  rectangular  sheets 
of  steel,  Fig.  264.  In  building 
up  the  core  out  of  the  stamp- 
ings care  is  taken  to  break 
joints  in  successive  layers.  The 
top  frame  is  then  put  on  and 
tightly  clamped  to  the  bottom, 
thus  compressing  and  holding 
the  core.  After  the  addition 
of  connection  board,  leads,  in- 
sulating bushings,  and  the  like, 
the  transformer  is  ready  for  its 
casing  or  tank. 

Fig.  264  shows  a  large  General  Electric  air-blast,  shell-type 
transformer  at  that  stage  of  construction  where  the  core  stampings 
are  being  built  up  around  the  coils,  the  coils  being  protected  by  thick 
strips  of  insulating  material.  In  this  air-blast  transformer,  air  passages 
or  ducts  are  provided  between  the  layers  of  the  coils,  and  at  intervals, 
between  the  core  stampings,  as 
shown  in  the  figure.  The  air 
for  cooling  the  transformer  is 
admitted  at  the  base,  and 
passes  vertically  through  the 
ducts  in  the  coils.  Air  is  also 
admitted  to  the  air  ducts  in  the 
core  through  a  damper  on  one 
side  of  the  transformer,  and  es- 
capes through  the  perforations  in  the  casing  on  the  opposite  side. 


Fig.  265. 


Sectional  View  of  Fort  Wayne  Shell- 
Type  Transformer 


Fig.  266. 


Sample  Coils  for  Fort  Wayne 
Transformer 


286 


ALTERNATING-CURRENT  MACHINERY 


Figs.  265  to  268  give  a  general  view  and  structural  details  of 
a  15-kw.  shell-type  transformer  manufactured  by  the  Fort  Wayne 
Electric  Works.  The  shape  of  the  core  stampings  is  shown  in  Fig. 
265,  which  is  a  section  through  the  transformer.  The  secondary 
winding,  as  seen  in  the  figure,  is  subdivided  into  four  sections,  and 
the  primary  into  two.  The  order  of  the  sections  as  arranged  in 
the  "window"  of  the  core  is:  One-fourth  total  secondary  turns, 
one-half  total  primary  turns,  one-fourth  secondary  turns,  one-fourth 


Fig.  267. 


Assembled  Core  and  Coils  for  Fort  Wayne 
Transformer 


secondary  turns,  one-half  primary  turns,  and  one-fourth  secondary 
turns.  This  intermixing  the  various  sections  reduces  greatly  mag- 
netic leakage,  and  thus  improves  regulation. 

Fig.  266  shows  a  group  of  pancake  coils  insulated  and  taped 
ijeady  to  be  assembled. 

Fig.  267  shows  the  core  and  coils  assembled.  The  four  leads 
on  the  left  are  from  the  secondary  coils,  and  the  six  leads  on  the 
right  are  the  four  primary  terminals  to  which  are  added  two  extra 
taps. 

Fig.  268  is  a  top  view  of  the  transformer  with  the  cover  re- 


ALTERNATING-CURRENT  MACHINERY  287 

moved  and  shows  how  the  leads  are  brought  up  to  the  connection 
board  and  out  of  the  case.  Each  of  the  two  primary  sections  is 
provided  with  what  is  called  a  ten  per  cent  tap.  That  is,  while  the 
normal  voltage  for  which  each  primary  section  is  wound  is  1,100 
volts,  by  using  the  ten  per  cent  taps,  it  is  possible  to  change  the 
ratio  of  transformation,  and  obtain  ratios  of  20  : 1, 19  : 1, 18  : 1, 10:1, 
9.5  : 1,  9:1,  5:1,  and  4.5  : 1,  according  to  the  connections  of  the 
primary  and  the  secondary  sections,  whether  in  series  or  in  parallel. 
The  advantage  of  these  ten  per  cent  taps  is  that  it  permits  a 
transformer  to  be  used  near  the  station  where  the  voltage  is  high 
or  at  the  end  of  a  feeder  where  the  voltage  may  be  five  or  ten  per 
cent  lower,  by  simply  changing  the  electrical  connections  at  the 
terminal  board  shown  in  Fig.  268. 


Fig.  268.     Top  View  of  Fort  Wayne  Transformer  with  Cover 
Removed  Showing  Arrangement  of  Lead  Wires 

Figs.  269  to  271  show  constructive  details  peculiar  to  very  large 
shell-type  transformers  which  are  usually  of  the  water-cooled  type. 
Fig.  269  represents  a  General  Electric  transformer  with  its  contain- 
ing tank  removed  to  show  the  method  of  suspending  the  core  and 
cooling  coils  from  the  cover.  The  cooling  coils  are  of  lap-welded 
wrought-iron  pipe  with  electrically  welded  joints.  Water  is  kept 
circulating  through  these  coils  in  order  that  the  oil  filling  the  case 
and  surrounding  the  transformer  may  be  kept  cool  while  the  trans- 
former is  in  operation.  The  core  and  the  coils  are  first  assembled 
after  which  they  are  tightly  clamped  and  suspended  from  the  cover 


288 


ALTERNATING-CURRENT  MACHINERY 


by  means  of  heavy  bolts,  as  shown  in  Fig.  269.  The  completed  trans- 
former filled  with  oil  may  be  easily  lifted  and  moved  about  by  means 
of  the  heavy  lifting  lugs  on  the  upper  side  of  the  cover. 

Some  of  the  advantages  of  this  cover  suspension  construction 
(a)  all  coil  terminals  are  brought  out  through  the  cover  and, 


are: 


Fig.  269.     General  Electric  Shell-Type  Water-Cooled 

Transformer — Tank  Removed  and  Cooling 

Coila  Shown  in  Section 


therefore,  are  not  interfered  with  when  the  transformer  is  removed 
from  its  tank,  (b)  The  terminal  board  at  the  top  is  accessible 
through  openings  in  the  cover  and  changes  in  electrical  connection 
jan  be  made  by  simply  raising  the  transformer  a  few  feet  out  of  the 
tank  by  a  crane,  without  drawing  off  any  oil.  Inspection  of  all 
parts  is  also  easily  and  quickly  made,  (c)  There  being  but  a  few 
bolts  to  loosen,  it  is  easy  to  remove  the  transformer  from  the  tank, 


ALTERNATING-CURRENT  MACHINERY 


289 


or  to  lift  tank  and  all  by  means  of  the  lugs  on  the  cover,  (d)  With 
this  construction  there  is  no  need  of  lowering  crane  hooks  or  chains 
into  the  tank,  thereby  avoiding  possible  danger  to  insulators  and 
to  coil  insulation. 

The  construction  of  water-cooled  transformers  is,  in  mechanical 


Fig.  270.     General  Electric  Water-Cqoled 
Transformer  Showing  Coils  in  Position 


and  electrical  design,  similar  to  the  oil-cooled  transformers,  the  only 
difference  being  in  the  tank.  In  the  water-cooled  type  the  tank  is 
built  up  of  heavy  boiler  plate  iron  riveted  to  a  cast-iron  base.  All 
joints  on  the  tank  are  heavily  riveted  and  thoroughly  caulked  to 
make  them  oil  tight. 

Three=Phase  Transformers.     During  the  past  few  years  the 
size  of  transformer  units  has  been  steadily  increasing  in  response 


290 


ALTERNATING-CURRENT  MACHINERY 


to  the  demand  from  the  power  companies  engaged  in  generating 
and  transmitting  electrical  power  on  an  enormous  scale.  Long- 
distance transmission  of  electrical  power  is  most  economically 
carried  out  by  the  three-phase  system,  which  is  the  standard  prac- 


Fig.  271.     General  Electric  Water-Cooled 
Transformer  Completely  Assembled 


tice.  These  conditions  have  caused  a  demand  for  large  three-phase 
transformers,  which  under  certain  conditions  are  to  be  preferred  to 
three  single-phase  transformers  of  the  same  aggregate  capacity, 
but  no  general  rule  can  be  given  as  to  the  relative  value  of  the  two 
types. 

The  advantages  of  the  three-phase  transformer  over  a 
group  of  three  single-phase  transformers  of  the  same  total  capa- 
city are: 


ALTERNATING-CURRENT  MACHINERY          291 

(1)  Lower  cost. 

(2)  Higher  efficiency 

(3)  Requires  less  floor  space. 

(4)  Has  less  weight. 

(5)  Connections  and  outside  wiring  very  much  simplified  as  only  three  primary 
and  three  secondary  leads  are  usually  brought  out. 

(6)  Lower  transportation  charges  and  cost  of  installation. 

(7)  Presents  a  symmetrical  and  compact  appearance. 

The  disadvantages  of  the  three-phase  type  are: 

(1)  Greater  cost  of  spare  units. 

(2)  Greater  derangement  of  service  in  case  of  break-down. 

(3)  Greater  cost  of  repairs. 

(4)  Reduced  capacity  obtainable  in  self-cooling  units. 

(5)  Greater  difficulty  in  bringing  out  taps  for  a  large  number  of  voltages. 

In  general  single-phase  transformers  are  preferable  where  only 
one  transformer  is  installed  and  where  the  expense  of  a  spare  trans- 
former would  not  be  warranted.  In  such  installations  the  burnout 
of  one  phase  of  a  three-phase  unit  would  cause  considerable  incon- 
venience for  the  reason  that  the  whole  transformer  would  have  to  be 
disconnected  from  the  circuit  before  repairs  could  be  made.  If, 
however,  single-phase  transformers  are  used,  the  damaged  trans- 
former can  be  cut  out  with  a  minimum  amount  of  trouble,  and  the 
other  two  transformers  can  be  operated  at  normal  temperature 
open  A  -  (or  V-) connected  at  58  per  cent  of  the  normal  capacity 
of  the  group  of  three  transformers,  until  the  third  unit  can 
be  replaced. 

With  a  three-phase  shell-type  transformer,  if  both  the  primary 
and  the  secondary  are  A-connected,  trouble  in  one  phase  will  not 
prevent  the  use  of  the  other  two  phases  in  open  delta.  By  short- 
circuiting  both  primary  and  secondary  of  the  defective  phase  and 
cutting  it  out  of  circuit,  the  magnetic  flux  in  that  section  is  entirely 
neutralized.  This  cannot  be  done,  however,  with  any  but  A-con- 
nected transformers.  Where  a  large  number  of  three-phase  trans- 
formers can  be  used,  it  is  generally  advisable  to  install  three-phase 
units. 

Three-phase  transformers  are  made  both  of  the  core  and  the 
shell  types,  according  to  circumstances.  Thus,  Fig.  272  shows  a 
three-phase  core-type  transformer  with  its  case  removed,  made  by 
the  General  Electric  Company.  A  three-legged  core  is  used,  each 
leg  being  wound  with  the  primary  and  the  secondary  coils  of  one 


292 


ALTERNATING-CURRENT  MACHINERY 


phase.  The  magnetic  circuit,  arranged  as  shown  in  Fig.  226,  is 
explained  on  page  47.  Since  the  weight  of  the  core  and  the  coils 
is  greater  than  in  the  single-phase  transformers,  the  core-clamps 
and  other  mechanical  parts  are  made  larger  and  stronger  while  two 
bolts  on  each  side,  instead  of  one,  are  used  to  support  it  from  the 
cover.  Three  leads  only  are  brought  out  of  the  cover  on  the  primary 
and  the  secondary  sides,  all  connections  being  made  on  the  inside 
of  the  tank. 

The  General  Electric  Company   has   built   three-phase  trans- 
formers of  the  shell  type  up  to  10,000  kilowatts  capacity,  for  100,000 

volts  primary  and  11,000  volts 
secondary,  and  a  frequency  of  60 
cycles.  They  are  now  building 
them  for  an  output  of  14,000  kilo- 
watts. 

Fig.  273  shows  a  three-phase 
shell-type  of  transformer  removed 
from  its  tank.  It  is  rated  at  1 ,800 
kilovolt-amperes,  48,000  volts 
and  25  cycles.  This  transformer, 
which  is  built  by  the  Westing- 
house  Company,  is  designed  to  be 
placed  in  a  tank  filled  with  oil 
which  is  kept  cool  by  water  circu- 
lating in  a  coil  of  brass  tubing  sur- 
rounding the  transformer  and 
below  the  surface  of  the  coil. 

Cooling  of  Transformers. 
Transformers  of  moderate  size 

have  large  radiating  surface  compared  with  their  losses.  Such 
transformers,  therefore,  can  radiate  the  heat  due  to  core  losses  and 
copper  losses  without  excessive  rise  of  temperature. 

A  transformer  which  is  twice  as  large  as  a  given  transformer 
in  length,  in  breadth,  and  in  thickness,  has  eight  times  as  much 
volume  but  only  four  times  as  much  radiating  surface  as  the  latter. 
The  large  transformer  having  nearly  eight  times  the  losses  and 
four  times  the  radiating  surface  of  the  smaller  transformer  would 
rise  to  a  much  higher  temperature  than  the  smaller  transformer  in 


Fig.  272.     General  Electric  Three-Phase 

Core-Type  Transformer — Case 

Removed 


ALTRENATING-CURRENT  MACHINERY  293 

order  to  radiate  the  heat  due  to  its  losses.  Large  transformers 
must,  therefore,  be  provided  with  special  means  for  cooling.  It  is 
much  cheaper  to  provide  special  cooling  devices  than  to  attempt  to 
make  the  transformer  large  enough  to  keep  cool  by  natural  radiation. 


Fig.  273.     Westinghouse  Three-Phase  Shell-Type 
Transformer — Tank  Removed 

Various  methods  of  cooling  are  adopted  in  practice,  ac- 
cording to  which  the  following  classification  of  transformers  may 
be  made: 

Self-cooling  dry  transformers. 
Self-cooling  oil-filled  transformers. 
Transformers  cooled  by  forced  current  of  air. 
Transformers  oil-filled  cooled  by  forced  current  of  water. 

Self-Cooling  Dry  Transformers.  These  transformers  are  usually 
of  small  output,  and  no  special  means  of  cooling  is  provided,  the 


294  ALTERNATING-CURRENT  MACHINERY 

natural  radiation  being  depended  upon  for  cooling.  Some  larger 
ones  up  to  5-  or  10-kilowatt  capacity  have  been  made  in  this  way, 
but  they  are  heavier  and  more  expensive  than  if  oil-cooled. 

Self-Cooling,  Oil-Filled  Transformers.  Transformers  of  this 
type  are  very  generally  employed,  the  entire  core  and  coils  being 
immersed  in  oil.  Transformers  are  practically  always  enclosed  in 
a  cast-iron  or  sheet-steel  case,  and  this  is  simply  filled  with  a  special 
high-grade  mineral  oil.  No  increase  in  cooling  surface  is  thereby 
secured,  but  the  natural  circulation  of  the  oil  tends  to  equalize  the 
temperature  of  the  various  parts,  and  carries  the  heat  to  the  case, 
from  which  it  is  radiated.  In  most  self-cooling  types,  the  case  is 
made  with  external  ribs  or  corrugations  to  increase  its  radiating 
surface.  The  large  volume  of  oil  also  absorbs  considerable  heat, 
so  that  the  temperature  rises  more  slowly.  Hence,  for  moderate 
periods  of  operation,  up  to  3  or  4  hours — which  is  ordinarily  sufficient 
in  electric  lighting — the  maximum  temperature  would  not  be  reached. 
Another  advantage  gained  by  this  arrangement  is  an  improvement  in 
insulation.  This  is  due  to  the  high  insulating  qualities  of  the  oil 
itself,  and  to  the  fact  that  a  disruptive  discharge  takes  place  through 
it  much  less  readily  than  through  the  air  that  it  displaces,  distances 
being  the  same.  This  arrangement  possesses,  moreover,  the  power 
of  self-repairing  any  break  in  the  insulation.  If  ordinary  materials, 
such  as  cloth  or  mica,  become  punctured,  they  lose  their  insulating 
properties,  and  the 'apparatus  cannot  be  used  until  the  fault  is  re- 
paired, which  ordinarily  involves  considerable  time  and  expense. 
On  the  other  hand,  if  oil  is  punctured,  it  tends  to  close  in  and  repair 
the  break,  unless  the  discharge  lasts  so  long  that  a  charring  occurs, 
which  may  make  a  permanent  conducting  path. 

The  chief  objection  to  the  use  of  oil  is  the  danger  of  fire.  If 
a  short-circuit  occurs  inside  the  transformer,  the  oil  may  be  thrown 
out  and  ignited  at  the  same  time;  or  a  fire  started  in  any  other  way 
might  be  made  far  more  disastrous  than  it  would  otherwise  be 
owing  to  the  presence  of  a  large  quantity  of  oil.  In  this  way,  several 
power  plants  have  been  destroyed  by  fire  with  large  loss  of  prop- 
erty. There  is  no  special  precaution  that  will  entirely  eliminate 
this  risk;  but  care  in  locating  such  transformers,  in  avoiding  over- 
heating, and  in  protecting  the  machines  by  effective  lightning  ar- 
resters., will  reduce  the  hazard. 


ALTERNATING-CURRENT  MACHINERY 


295 


Oil-cooled  transformers  can  be  built  for  any  voltage  and  to 
almost  any  size,  although  the  economical  maximum  limit  in  output 
is  reached  at  about  500  kilowatts. 


^^j^^^^^?^^ 

Fig.  274.     Bank  of  Air-Blast  Transformers  Showing  Arrangement  of  Air  Chamber 

Air-Blast  Transformers.  These  transformers  are  now  com- 
monly employed,  and  have  advantages  over  those  of  the  oil-cooled 
type,  in  that  the  danger  of  fire  is  avoided,  and  the  cooling  effect 
can  be  regulated  in  accordance  with  working  conditions.  They  are 
so  constructed  that  air  can  circulate  through  and  around  the  core 
and  coils,  the  ventilation  being  forced  by  a  blower  driven  by  a 
motor.  A  transformer  of  100-kw.  capacity  requires  about  450 
cubic  feet  of  air  per  minute 
at  a  pressure  of  0.5  ounce 
per  square  inch,  the  power 
consumed  by  the  blower  set 
being  less  than  one  per  cent  of 
the  full-load  output  of  the 
transformer.  The  flow  of  air 
is  controlled  by  dampers;  and 
the  proper  amount  of  air  can 
be  determined  from  its  tem- 
perature as  it  issues  from  the 
top;  ordinarily  this  tempera- 
ture should  not  be  more  than 
20°C.  above  the  temperature 


//  AirCHamberY 
7  \ 


Fig.  -275.     Section  of  Air-Blast  Transformer 
Showing  Circulating  Currents 


of  the  room. 

Fig.  274  shows  a  bank  of  six  large  air-blast  transformers  sup- 
ported on  !  beams  over  an  air  chamber  supplied  with  air  from  a 
fan  blower.  The  air  passes  in  at  the  bottom  of  each  transformer 


296  ALTERNATING-CURRENT  MACHINERY 

case,  penetrates  through  ducts  in  the  core  and  coils,  and  passes  out 
at  the  top  of  the  case,  as  shown  in  Fig.  275. 

The  process  of  building  up  the  core  of  a  shell-type  air-blast 
transformer  is  illustrated  in  Fig.  264.  The  air-blast  type  is  used  for 
moderate  voltages  where  cooling  water  is  expensive  or  not  available, 
and  is  built  for  voltages  up  to  33,000  in  sizes  up  to  5,000  kilowatts. 
The  limiting  voltage  for  this  type  is  determined  by  the  excessive 
thickness  of  the  solid  insulation  needed  and  the  consequent  difficulty 
in  radiating  heat  from  the  copper.  For  voltages  above  33,000,  the 
oil-insulated  water-cooled  type  is,  therefore,  recommended. 

Water-Cooled  Transformers.  Transformers  in  which  water  is 
used  for  cooling  are  always  immersed  in  oil.  In  an  oil-insulated 
water-cooled  transformer,  all  the  heat  generated  in  the  iron  and 
copper,  except  a  small  amount  radiated  from  the  surface  of  the  case, 
is  dissipated  by  means  of  water  flowing  through  coils  of  pipe  placed 
under  the  oil  near  the  surface.  As  the  copper  and  iron  become 
heated,  the  heat  is  transferred  to  the  oil  coming  into  contact  with 
their  surfaces.  As  the  oil  is  heated  up,  convection  currents  are  pro- 
duced by  the  hot  oil  rising  from  the  transformer  and  flowing  over 
towards  the  sides  of  the  case;  here  it  comes  into  contact  with  the 
surface  of  the  cooling  coils,  giving  up  its  heat  to  them  and  then 
sinks  along  the  sides  of  the  case  to  the  bottom  where  it  is  ready  to 
be  heated  up  again  and  repeat  the  cycle.  This  method  of  water 
cooling  is  so  effective  that  very  little  heat  is  dissipated  from  the 
tank  and  there  is  nothing  to  be  gained  by  corrugations.  From  this 
it  is  seen  that  the  cooling  coils  form  a  very  important  part  of  the 
oil-insulated  water-cooled  transformer  and  that,  if  for  any  reason 
they  should  fail  to  perform  their  work  of  carrying  away  the  heat  or 
should  develop  a  leak,  the  transformer  may  be  seriously  damaged 
or  even  destroyed. 

Water  cooling  is  at  present  the  most  effective  method  for 
dissipating  the  heat  generated  in  very  large  units  and  where- 
ever  the  voltages  exceed  33,000  volts.  It  is  very  convenient  for 
water-power  plants,  the  supply  of  water  being  at  hand;  but  where 
a  natural  flow  is  not  available,  and  pumps  or  city  water  mains  have 
to  be  utilized,  the  expense  may  be  prohibitive.  * 

*In  any  of  these  types  of  transformers  depending  upon  forced  circulation  of  air  or 
water,  it  is  vitally' important  to  avoid  any  stoppage  of  the  flow,  as  this  is  likely  to  cause  a  burn- 
out  of  the  transformer  coils. 


ALTERNATING-CURRENT  MACHINERY 


297 


Series  or  Current  Transformers.  The  chief  difference  between 
the  series  (current)  and  the  shunt  (voltage)  transformer  is,  as  the 
name  implies,  in  the  manner  of  connecting  it  in  the  circuit.  The 
series  transformer  has  its  primary  coil  in  series  with  the  line,  while 
the  shunt  transformer  has  its  primary  shunted  across  the  line  wires. 
In  the  latter  the  primary  voltage  is  determined  by  that  of  the  main 
circuit,  and  the  primary  current  is  determined  by  the  impedance 
of  the  transformer,  varying  according  to  the  secondary  load.  In 
the  series  transformer,  on  the  other  hand,  the  primary  current  is 
determined  by  the  current  in  the  main  circuit,  and  the  primary  vol- 
tage is  merely  the  drop  across  the  primary  terminals  due  to  the 
primary  impedance.  The  total  impedance  of  the  secondary  circuit 
being  normally  constant,  the  change  in  load  is  due  to  a  simultaneous 
change  in  the  primary  current 
and  voltage.  In  the  shunt 
(voltage)  transformer  the  act- 
ual ratio  of  the  primary  to  the 
secondary  current  is  of  minor 
consequence,  whereas  a  con- 
stant ratio  of  voltages  (espe- 
cially in  the  so-called  potential 
transformers  used  in  connec- 
tion with  voltmeters  and  watt- 
meters) is  of  the  highest  impor- 
tance. In  the  design  of  a  series 
transformer,  the  questions  of 
constant-voltage  ratio  and  of 
high  efficiency  receive  no  attention,  whereas  the  matter  of  securing  a 
definite  ratio  of  secondary  to  primary  amperes  receives  the  most  care- 
ful attention. 

It  was  pointed  out  on  page  249  that  a  transformer  intended  to 
give  an  accurately  fixed  ratio  between  primary  and  secondary  cur- 
rents must  be  so  designed  as  to  have  a  very  small  magnetizing 
current.  This  is  accomplished  by  using  a  very  low  magnetic  flux 
density  in  the  iron  core,  so  as  to  reduce  the  watts  of  core  loss  to  a 
minimum.  The  magnetizing  current  is  also  reduced  by  using  iron- 
core  plates  of  high  permeability  and  with  no  breaks  or  joints  in  the 
magnetic  circuit. 


Fig.  276. 


Portable  Current  Transformer  Showing 
Primary  Winding  Terminals 


298 


ALTERNATING-CURRENT  MACHINERY 


The  series  transformer  is  mostly  employed  for  insulating  an 
ammeter,  a  current  relay,  or  series  coil  of  a  wattmeter  or  watt-hour 
meter  from  a  high  voltage  circuit,  or  for  reducing  the  line  current 
to  a  value  suited  for  these  instruments.  The  secondary  coils  of  such 
current  transformers  are  usually  wound  for  five  amperes. 

Figs.  276  and  277  show  two  commercial  types  of  portable  cur- 
rent transformers  made  by  the  General  Electric  Company.  Both 
types  have  cores  built  up  of  closed  ring-shaped  stampings  of  thin 
sheet  steel.  The  primary  winding  of  the  transformer  shown  in  Fig. 
276  is  divided  into  four  coils,  both  ends  of  each  coil  being  brought 
out  at  one  end  and  connected  to  suitable  terminal  blocks.  By  con- 
necting- these  coils  in  series,  series-parallel,  or  in  parallel,  three 
different  current  ratios  are  obtained,  the  standard  ratings  being 

200-100-50  amperes  primary  with  5 
amperes  secondary.  The  secondary  is 
wound  as  one  coil,  the  terminals  of 
which  are  brought  out  at  the  top. 
The  type  shown  in  Fig.  277  has  no 
regular  primary  winding,  but  the  core 
is  provided  with  an  opening  through 
which  a  cable  carrying  the  current  to  be 
measured  may  be  passed  one  or  more 
times  to  make  the  primary  winding. 
The  ratio  of  transformation  depends 
upon  the  number  of  times  the  cable  is 
made  to  pass  through  this  opening. 
This  form  of  current  transformer  is 
designed  to  have  1,000  ampere  turns  at 
full-load  rating.  The  standard  transformer  has  a  5-ampere  secondary 
winding  giving  a  ratio  with  one  primary  turn  passing  through  the  center 
of  the  core  of  100 : 5,  or  200 : 1 .  If  the  cable  is  passed  through  the  open- 
ing twice,  the  ratio  is  500 : 5,  or  100 : 1,  etc.  Both  types  are  rated  at  40 
watts,  and  may  be  used  on  circuits  the  voltage  of  which  does  not  exceed 
2,500  volts. 

The  instrument,  such  as  an  alternating-current  ammeter,  to 
be  used  with  these  transformers,  is  connected  directly  in  series  with 
the  secondary  coil.  If  the  ratio  of  transformation  is  200 : 5  and  the 
ammeter  indicates  5  amperes,  the  actual  value  of  the  current  passing 


Fig.  277.     Portable  Current  Trans- 
former without  Regular  Primary 
Windings 


ALTERNATING-CURRENT  MACHINERY 


299 


in  the  line  and  through  the  primary  will  be  200  amperes.  In  Fig. 
217  the  coil  A,  in  series,  with  the  secondary  coil  S*,  may  represent 
the  switchboard  ammeter. 

The  question  may  arise  as  to  why  the  switchboard  ammeter  is 
not  connected  as  a  shunt  across  the  terminals  of  a  low-resistance 
link  inserted  in  the  main  circuit  whose  current  is  to  be  measured, 
exactly  as  in  the  case  of  switchboard  ammeters  for  direct  currents._ 
There  are  two  reasons  why  this  arrangement  would  not  be  permissible: 

(a)  An    alternating    current    does    not    divide    between    two 
branches  of  a  circuit  in  inverse  proportion  to  the  resistances  of  the 
branches,  when  either  branch  has  inductance.    Therefore,  the  reading 
of  a  shunted  alternating-current 

ammeter  cannot  be  multiplied 
by  a  constant  factor  to  give  the 
total  current  in  the  main  circuit. 

(b)  It  is   objectionable   to 
have   the   ammeter  in  electrical 
connection      with     high-voltage 
mains.     The  use  of   the   series 
transformer  is,  therefore,  prefer- 
able   on   the    grounds    of   both 
accuracy  and  safety. 

Constant=Current  Transf  orm= 
ers.  The  constant-current  trans- 
former is  a  transformer  spe- 
cially designed  to  take  a  nearly 
constant  current  at  varying  angles  of  lag  from  constant-voltage 
mains,  and  to  deliver  a  constant  current  from  its  secondary  coil  to 
a  receiving  circuit  of  variable  resistance.  The  action  of  this  trans- 
former is  as  follows:  The  primary  coil  P  and  the  secondary  coil  S 
of  the  transformer  surround  a  long,  laminated-iron  core,  as  shown 
in  Fig.  278.  This  core,  and  the  yokes  at  the  top,  bottom,  and  sides, 
form  a  double  magnetic  circuit.  The  magnetic  flux  <f>,  which  at  a 
given  instant  passes  through  the  primary  coil,  flows  partly  through 
the  secondary  coil  as  the  useful  flux  U,  and  partly  leaks  across  be- 
tween the  primary  and  secondary  coils  as  the  leakage  flux  L. 

The  leakage  flux  L,  in  flowing  across  the  air  spaces  from  core  to 

*The  electrical  connections  for  a  series  transformer  have  been  described  on  page  249. 


Fig.  278.     Magnetic  Circuit  in  Constant- 
Current  Transformer 


300 


ALTERNATING-CURRENT  MACHINERY 


TABLE  VIII 
Constant=Current  Transformer  Data* 


PRIMARY 

VOLTS 

NUMBER  OP 
LAMPS 

VOLTS  PER 
LAMP 

SECONDARY 
"  VOLTS 

SECONDARY 
AMPERES 

2,200 

50 

76.6 

3,830 

6.60 

2,200 

40 

77.6 

3,105 

6.70 

2,200 

30 

77.2 

2,315 

6.67 

2,200 

25 

81.4 

2,035 

6.65 

2,200 

20 

84.2 

1,685 

6.65 

2,200 

15 

86.0 

1,290 

6.65 

second. 


*Test  of  a  50-light,  6.6-ampere  constant-current  transformer,    freqxiency  60  cycles  per 


yokes,  constitutes  an  intense  magnetic  field  which  pushes  up  on  the 
secondary  coil.  The  secondary  coil  is  suspended,  and  partly  counter- 
balanced by  a  weight,  so  that  the  upward  push  of  the  leakage  flux 
just  suffices  to  sustain  the  coil.  If  the  resistance  of  the  secondary 
receiving  circuit  is  increased,  the  immediate  result  is  to  reduce 
the  secondary  current  below  its  normal  value,  which  lessens  the 
upward  push  of  the  leakage  flux  on  the  secondary  coil.  The  sec- 
ondary coil  then,  owing  to  the  unbalanced  action  of  the  weight, 
moves  down  towards  P;  the  leakage  flux  is  lessened  in  amount 
and  the  useful  flux  U  is  increased  in  amount.  This  increase  of 
useful  flux  increases  the  induced  electromotive  force  in  S;  and  the 
downward  movement  of  S  continues  until  the  induced  electromo- 
tive force  in  S  is  large  enough  to  produce  the  normal  value  of  the 
current  through  the  increased  secondary  resistance. 

Similarly,  a  decrease  of  resistance  of  the  secondary  receiving 
circuit  causes  a  momentary  increase  of  secondary  current  which 
increases  the  upward  push  on  the  secondary  coil.  This  coil  moves 
upwards  until  the  secondary  current  is  reduced  to  the  normal  value. 

Table  VIII  shows  the  approximate  constancy  of  secondary  cur- 
rent in  a  constant-current  transformer  supplying  current  to  a 
varying  number  of  arc  lamps  connected  in  series  to  its  secondary  coil. 

With  50  lamps,  the  primary  current  is  14.83  amperes;  arid 
with  20  lamps,  is  14.78  amperes.  In  the  latter  case,  the  primary 
current  lags  71  degrees  behind  the  primary  applied  voltage;  the 
power  factor  corresponding  to  this  angle  is  0.326;  and  the  power 
received  from  the  mains  is  2,200  X  14.78  X  0.326=10,600  watts,  or 


ALTERNATING-CURRENT  MACHINERY  301 

530  watts  per  lamp.  With  40  lamps,  the  primary  current  lags  55 
degrees  behind  the  primary  applied  voltage;  the  power  factor  corre- 
sponding to  this  angle  of  lag  is  0.574;  and  the  power  received  from 
the  mains  is  2,200X14.81X0.574  which  is  equal  to  18,800  watts, 
or  470  watts  per  lamp.  The  efficiency  of  the  transformer  with  a 
50-lamp  load  was  93.9  per  cent,  and  with  a  20-lamp  load  it  was 
85.7  per  cent.  The  power  factor  of  the  system  as  a  whole  varies 


Fig.  279.     Mechanism  of  General  Electric  Two-Coil  Constant-Current  Transformer 

from  72  per  cent  to  76  per  cent  at  full  load,  decreasing  considerably 
at  light  loads. 

Fig.  279  is  a  general  view  of  the  mechanism  of  the  two-coil 
(one  primary,  and  one  secondary),  constant-current  transformer 
of  the  General  Electric  Company;  and  Fig.  280  shows  the  trans- 
former in  its  containing  case. 

The  mechanism  of  the  transformer  is  surrounded  by  a  corru- 
gated sheet  iron  casing  designed  primarily  for  the  protection  of  the 


302  ALTERNATING-CURRENT  MACHINERY 

coils.  The  casing  is  enclosed  by  a  cast-iron  base  and  top  which  are 
provided  with  ample  openings  for  the  proper  ventilation  of  the 
transformer. 

Within  the  working  limits,  the  magnetic  repulsion  between 
the  fixed  and  moving  coils  of  the  system  for  a  given  position  is  pro- 
portional to  the  current  flowing  in  the  coils,  which  makes  the 


Fig.  280.     General  Electric  Two-Coil  Transformer  Completely 
Assembled 


transformer  capable  of  being  adjusted,  therefore,  so  as  to  main- 
tain any  current,  simply  by  changing  the  amount  of  counter- 
weight. 

In  transformers  up  to  and  including  50-lamp  capacity  having 
but  one  movable  coil — the  secondary,  as  in  Fig.  279 — the  counter- 
weight is  equal  to  the  weight  of  the  coil  less  the  electrical  repul- 
sion; and  a  reduction  in  the  counterweight  will  produce  an  increase 


ALTERNATING-CURRENT  MACHINERY  303 

* 

in  the  current.  A  lever  is  supported  by  knife-edge  bearings  on 
hardened  steel  tables  which  are  clamped  to  the  top  of  the  core.  To 
one  end  of  this  lever  are  secured  two  fixed  arcs  to  which  are  attached 
two  cables  which  support  the  movable  secondary  coil.  At  the  outer 
end  of  the  lever  an  adjustable  arc  carries  a  counterweight  suspended 
by  a  cable. 

In  transformers  designed  to  supply  75  and  100  lights,  having 
two  primary  and  two  secondary  coils,  the  movable  secondary  cotls 
are  balanced  one  against  the  other  by  a  system  of  double-rocker  arms 
supported  on  knife  edges.  The  weight  necessary  to  balance  the 
repulsion  between  the  primary  and  the  secondary  coils  is  carried  on  a 
small  auxiliary  lever.  In  this  case,  a  decrease  in  the  counterweight 
is  followed  by  a  decrease  in  the  current. 

The  arc  on  the  counterweight  lever  is  made  adjustable  because 
the  repulsion  exerted  by  a  given  current  flowing  in  the  coils  is  not  the 
same  for  all  positions  of  the  coils,  being  greater  when  the  primaries 
and  secondaries  are  close  together  and  less  when  the  primaries  are 
separated.  By  means  of  the  adjustable  arc,  the  effective  radius  of 
the  balancing  weight  is  made  to  change  as  the  coils  move  through 
their  working  range.  When  the  primary  and  the  secondary  coils 
are  separated  by  the  maximum  distance,  the  resultant  force  which 
tends  to  attract  them  to  each  other  should  be  less  than  when  they 
are  close  together. 

Regulation.  When  current  flows  in  the  primary  and  secondary 
coils,  the  mutual  repelling  forces  separate  the  coils  until  equilib- 
rium is  restored.  The  current  corresponding  to  the  position  of 
equilibrium  may  be  adjusted  by  changes  in  the  counterweights,  and 
the  coils  will  then  always  take  such  a  position  as  will  maintain  that 
current  constant  in  the  secondary  coils,  regardless  of  the  external 
resistances  to  which  the  coils  are  connected.  With  any  current  less 
than  normal,  the  repelling  force  diminishes,  and  the  primary  and 
secondary  coils  approach  each  other,  thus  restoring  normal  current. 
As  soon  as  the  secondary  current  exceeds  normal,  the  resultant  pull 
exerted  by  the  counterweight  and  coils  is  overcome,  and  the  sec- 
ondary coil  moves  away  from  the  primary,  again  restoring  normal 
current.  Transformers  of  this  design  can  be  made  to  maintain 
constant  current  even  more  accurately  than  the  constant-voltage 
transformer  maintains  uniform  voltage. 


304 


ALTERNATING-CURRENT  MACHINERY 


In  Fig.  281  on  the  left  is  shown  the  diagram  of  electrical  con- 
nections for  a  constant-current  transformer  for  either  5,  35,  or  50 
lights,  and  on  the  right,  the  diagram  for  the  75-  or  100-light  trans- 
former. The  75-  and  100-light  transformers  are  furnished  with  two 
primary  coils  which  may  be  connected  in  series  for  2,200  volts  or  in 
parallel  for  1,100  volts;  for  example,  connecting  B  to  C  puts  the  two 
primary  coils  in  series,  and  connecting  B  to  D  and  C  to  A  puts 
them  in  parallel  for  1,100  volts.  In  these  larger  transformers  there 
are  two  secondary  coils  also,  and  leads  are  arranged  so  that  the 


/•          *v 


Contact  next  to  Panel 
Contact  farthest  from  Panel 
ightniny  Arrester 


(When  Current  Transformer  is  supplied  connect 


\  as  shown.  Otherwise  conned  from  fti'.g 
\toAmmeter  and  from  Ammeter  to  Lamps. 


Ammeter  JacA^s 


Q)O/>e/?  Circulating  Plugr  Jtyitcn 

Constant  Current  Transformer 

Wattmeter 

Current  Transformer 

-  Th/j  dppardtus  furnished  only  when 
sub-base.  i<3  supplied 

Fuse  —? 

©  Plug  Switch 

^IC  Source  A.  &  Sourc e 

Fig.  281.     Connections  for  Constant-Current  Transformer  for  5,  35,  50,  75,  and  100  Lights 


-  tiooYolts  connect  BtoDandCtoA 

•  2200  falts  connect  £toC. 


lamps  can  be  divided  between  two  circuits   by  means  of  multi- 
circuit connections,  as  shown  in  Fig.  281  on  the  right. 

Open-circuiting  plug  switches  are  used  to  disconnect  the  line 
from  the  secondary  of  the  transformer  when  testing  for  a  ground 
or  an  open  circuit.  These  are  also  used  to  disconnect  one  of  the 
circuits  of  a  multi-circuit  transformer  in  order  that  it  may  be 
repaired  without  interrupting  the  other  circuit.  The  ammeter 
jacks,  which  are  provided,  are  used  for  the  purpose  of  enabling 
one  ammeter  to  measure  the  current  in  more  than  one  circuit.  By 


ALTERNATING-CURRENT  MACHINERY 


305 


inserting  the  plug  in  any  ammeter  jack,  that  particular  ammeter 
is  connected  in  series  with  that  circuit. 

The  primary  windings  are  provided  with  fuses  which  are  made 
part  of  the  primary  plug  switch  and  are  mounted  on  the  back  of 


Fig.  282.     Westinghouse  Single-Pole  Primary  Fuse  Block 

the  panel.  They  are  of  the  tube  expulsion  type,  depending  on  the 
expulsive  force  exerted  by  the  gases  formed  by  melting  of  the  fuse. 
This  force  is  sufficient  to  blow  out  any  arc  which  tends  to  form 
within  the  narrow  tube  in  which  the  fuse  is  located. 

Transformer  Fuse  Blocks.     The  safety  fuse  links  designed  to 
protect  a  transformer  from  burn-out  in  case  of  short-circuit,  are 
usually  placed  in  circuit  with  both  primary  and  secondary  coils. 
The  fuse  links  connected  in  circuit  with  the 
high-voltage  coil  are  usually  encased  in  a 
porcelain  tube  which  encloses  the  arc  that  is 
formed  when  the  fuse  melts;  and  the  expan- 
sion of  the  highly  heated  vapors  in  the  tube 
extinguishes  the  arc  by  what  is  called  "expul- 
sive" action. 

The  tubes  containing  the  fuses  are  usu- 
ally provided  with  brass  terminals  for  the 
fuses,  the  terminals  projecting  as  blades  from 
one  side  of  the  tube.  With  fuse  and  termi- 
nals complete,  the  tube  is  pushed  home  in  a 

receptacle  containing  metal  spring  clips  that  receive  the  fuse  terminals 
somewhat  after  the  manner  of  an  ordinary  knife-switch. 


Fig.  283.     Fort  Wayne  Sin- 
gle-Pole Primary  Fuse 
Block 


306  ALTERNATING-CURRENT  MACHINERY 

When  the  fuse  melts  or  "blows,"  the  tube  carrying  the  fuse 
and  terminals  is  withdrawn  from  the  receptacle.     A  new  fuse  may 


Fig.  284.     Method  of  Connecting  Wires  to 
Primary  Fuse  Block  on  Poles  Before 
Current  Passes  to  Transformer 

then  be  put  in  place  without  danger  to  the  attendant,  after  which 
the  tube  and  fuse  may  be  replaced  in  the  receptacle. 

In  small  transformers  for  moderate  voltages,  the  fuse  recep- 
tacles sometimes  form  part  of  the  containing  case;  but  in  large 
transformers  the  fuse  receptacles  are,  as  a  general  rule,  entirely 


ALTERNATING-CURRENT  MACHINERY 


307 


separate  from  the  transformer,  and  are  mounted  at  any  convenient 
point  near  the  transformer,  for  example,  on  a  cross-arm  or  on  the 
pole  where  the  transformer  is  placed.  In  the  case  of  transformers 
for  use  out-of-doors,  the  fuse  receptacles  always  consist  of  water- 
proof cases  of  cast  iron  or  porcelain,  usually  the  latter. 

Fig.  282  shows  the  type  of  single-pole  primary  fuse  block  fur- 
nished by  the  Westinghouse  Electric  Company.  The  block  is  made 
of  porcelain,  finished  in  black, 
and  is  weatherproof.  The  upper 
portion  of  the  cut-out  contains 
the  stationary  contacts  which  are 
deeply  recessed  in  the  porcelain 
and  are  well  separated  from  each 
other.  The  contacts  are  so  con- 
structed that  the  plug  is  held  se- 
curely in  place  by  giving  it  a  par- 
tial turn  after  inserting  it .  When 
the  plug  is  in  position,  the  fuse  is 
in  sight,  so  that  its  condition  can 
be  easily  noted  without  incurring 
the  danger  of  opening  the  primary 
circuit  by  pulling  out  the  fuse  plug 
while  the  fuse  is  still  intact  and 
the  transformer  is  under  load. 

Fig.  283  shows  a  single-pole 
fuse-plug  block  made  by  the  Fort 
Wayne  Electric  Works  for  pro-  c 
tecting  primary  circuits.  The 
block  and  plug  are  all  porcelain, 
finished  in  black,  and  serve  as 


-a   ] 


Fig.  285. 


Sample  Mounting  for  Out-Door 
Transformers 


both  a  primary  switch  and  a  cut- 
out.  The  block  is  mounted  on  the 

upper  cross-arm  of  the  pole  and  is  shaped  so  as  to  be  used  as  an 
insulator.  The  wires  may  be  brought  directly  from  the  line  to  the 
cut-out,  and  thence  to  the  primary  winding  of  the  transformer 
without  other  support,  as  shown  in  Fig.  284. 

The  plug  and  cavity  in  the  fuse  block  are  elliptical  in  section. 
The  plug,  shown  at  the  bottom  of  the  block  in  Fig.  283,  is  inserted 


308  ALTERNATING-CURRENT  MACHINERY 

concentrically  with  the  cavity  and  by  a  quarter  turn  the  contact 
blade  is  forced  between  bronze  spring  terminal  clips,  and  the  major 
axis  of  the  elliptical  section  of  the  plug  brought  into  line  with  the 
minor  axis  of  the  cavity.  The  result  is  that  the  cavity  is  divided 
into  two  parts  separated  by  the  plug  which  tightly  fits  the  cavity. 
The  fuse  lies  in  a  groove  in  the  plug  with  its  ends  held  under  screws 
on  the  plug  blades.  The  groove  is  long  enough  to  prevent  arcing, 
and  danger  of  shock  is  prevented  by  the  insulated  rim  of  the  plug. 
The  plug  is  firmly,  held  in  place  by  the  spring  terminal  clips. 

Mounting  of  Outdoor  Transformers.  Fig.  285  shows  a  trans- 
former in  its  water-tight  containing  case,  mounted  on  a  pole,  with 
its  primary  coil  connected  through  a  double-pole  fuse  block  to 
1,100-volt  mains,  and  its  two-coil  secondary  connected  to  three- 
wire  mains  for  supplying  incandescent  lamps  in  a  near-by  building. 
Two  suspension  hooks  A,  made  of  heavy  strap  iron,  are  attached 
to  the  back  of  the  transformer  case  by  slipping  the  bolt-heads  B 
into  the  sockets  C,  after  which  the  nuts  are  screwed  tight. 

The  transformer  is  hoisted  to  its  position  on  the  building  or 
cross-arm  by  means  of  a  rope  or  chain,  which  is  slipped  over  the 
two  hoisting  lugs  D.  When  the  transformer  is  hoisted  into  posi- 
tion, the  suspension  hooks  are  slipped  over  the  cross-arm,  and 
screwed  fast  by  means  of  lag  screws.  The  lead  wires  from  the 
primary  coil  are  connected  to  the  lower  fuse-box  leads,  and  the 
upper  fuse-box  leads  are  connected  to  the  mains.  The  wires  from 
the  secondary  coils  are  led  into  the  building  where  the  secondary 
fuses  are  placed.  It  is  now  more  usual  to  install  two  single-pole 
fuse  blocks  as  illustrated  in  Fig.  284,  than  one  double-pole  block 
as  shown  in  Fig.  285. 

TRANSFORMER  TESTS 

Heat  Test.  The  simplest  method  of  performing  this  test  is  to 
connect  the  primary  co'il  of  the  transformer  to  mains  giving  the 
rated  voltage  and  frequency  of  the  transformer,  and  to  load  the 
secondary  with  a  bank  of  lamps  or  a  water  rheostat,  adjusting  the 
resistance  so  as  to  get  rated  full-load  current  from  the  transformer. 
The  run  should  be  continued  until  an  approximately  constant  tem- 
perature is  reached. 

The  objection  to  loading  the  transformer  in  the  way  described 


ALTERNATING-CURRENT  MACHINERY 


309 


is  that  it  requires  taking  the  full  rated  power  from  the  transformer, 
which  power,  therefore,  is  usually  wasted.  If  two  transformers  of 
the  same  voltage  and  rated  capacity  are  available,  the  test  may 
be  made  on  the  two  simultaneously  by  what  is  known  as  the  motor- 
generator  method,  as  follows:  t 

The  two  secondaries,  Fig.  286,  connected  in  parallel,  are  excited  from 
a  low-voltage  circuit  A  at  normal  voltage  and  frequency;  consequently  normal 
voltage  is  induced  in  each  primary  winding.  The  two  primaries  are  connected, 
in  series,  but  in  such  a  way  as  to  oppose  each  other.  The  resultant  voltage 
between  the  points  a  and  b  will  then  be  zero,  notwithstanding  the  fact  that 
full  voltage  exists  between  the  terminals  of  each  transformer  secondary. 
Therefore,  if  the  points  a  and  6  be  joined  together  no  current  will  flow.  If. 
however,  instead  of  being  joined,  these  terminals  are  connected  to  the  ter- 
minals of  the  circuit  C,  any  voltage  impressed  at  C  will  produce  a  current  in 
the  circuit  of  the  primary  coils  independent  of  the  voltage  existing  in  each  of 


TransformerNo.z. 

Fig.  286.     Connections  for  Transformer  Heat  Test 

the  primary  coils.  Since  each  transformer  is  in  effect  short-circuited  by  the 
other,  it  follows  that  approximately  twice  the  impedance  voltage*  of  one 
transformer  impressed  at  C  will  cause  full-load  current  to  flow  through  the 
primaries  and  secondaries  of  both.  Under  these  conditions,  the  transformers 
will  run  at  full  load,  while  the  total  energy  required  for  the  test  amounts  to 
merely  the  losses  in  the  two.  The  circuit  A  supplies  the  excitation  current 
and  core  losses,  the  circuit  C  the  full-load  current  and  copper  losses. 

The  auxiliary  electromotive  force  impressed  at  C  may  be  derived  from 
the  same  source  as  the  electromotive  force  at  A,  by  means  of  a  transformer. 


*The  impedance  voltage  of  a  transformer  is  the  "electromotive  force  which  must  be 
applied  to  the  primary  coil  to  produce  full-load  current  in  both  coils  when  the  secondary 
coil  is  short-circuited.  This  voltage  is  from  2  per  cent  to  6  per  cent  of  the  rated  full-load 
primary  voltage.  See  page  312. 


310  ALTERNATING-CURRENT  MACHINERY 

A  regulating  resistance  must  be  connected  in  series  with  it  to  allow  adjust- 
ment of  the  electromotive  force  at  C  until  the  ammeter  registers  full-load  current. 

The  important  temperatures  to  be  observed  are  those  of  the  coils,  core, 
and  room.  The  temperature  of  the  case  and  of  the  oil  may  be  observed  as 
checks.  The  determination  of  the  temperature  of  the  coil  may  be  made  by 
thermometer  or  by  measurement  of  resistance.  If  a  transformer  has  remained 
in  a  room  of  constant  temperature  many  hours,  so  that  the  temperature  is 
approximately  uniform  throughout,  thermometer  measurement  indicates 
quite  accurately  the  temperature  of  the  windings.  If,  however,  the  transformer 
is  radiating  heat,  as  during  the  heat  run,  the  actual  temperature  of  the  copper 
coils  will  be  much  greater  than  the  temperature  of  surface  insulation. 

If  we  know  the  "cold"  resistance,  as  measured  under  the  first  of  the 
above  conditions,  and  the  temperature  of  the  coil  at  the  time  of  measurement, 
we  have  a  means  of  finding  the  "hot"  temperature  of  the  coil  by  measuring 
its  "hot"  resistance.  The  rise  in  temperature  above  the  temperature  at  which 
the  "cold"  resistance  was  measured,  may  be  determined  from  the  equation 


r  =  (238.1 


in  which  Rt  is  the  "cold"  resistance,  Rt+r  is  the  "hot"  resistance,  i  is  the 
initial  temperature,  and  r  is  the  rise  in  the  temperature  expressed  in  degrees 
centigrade.  The  temperature  coefficient  for  commercial  copper  wire  is  taken 
at  0.0042. 

If  the  room  temperature  differs  from  25°C.  the  observed  rise  in  tempera- 
ture should  be  corrected  by  0.5  per  cent  for  each  degree  centigrade.  Thus, 
with  a  room  temperature  of  35°  C.  the  observed  rise  should  be  decreased  by 
5  per  cent;  and  with  a  room  temperature  of  15°  C.,  the  observed  rise  should 
be  increased  by  5  per  cent. 

Core=Loss  and  Exciting=Current  Test.  For  this  test,  the  trans- 
former is  connected  as  shown  in  the  diagram,  Fig.  287,  the  primary 
being  left  on  open  circuit.  Theoretically  the  test  may  be  carried 
out  with  either  coil  connected  to  mains  of  the  proper  voltage  and 
frequency.  In  practice,  however,  it  is  better,  from  the  standpoint 
both  of  convenience  and  of  safety,  to  connect  the  secondary  coil. 

The  electromotive  force  is  adjusted  by  means  of  the  variable 
resistance  until  the  voltmeter  indicates  rated  secondary  voltage. 
The  ammeter  then  indicates  the  exciting  or  no-load  current;  and 
the  wattmeter  indicates,  very  closely,  the  core  loss, 

Resistance  of  Coils.  The  resistance  of  a  coil  of  a  transformer 
may  be  measured  by  the  ordinary  drop-of-voltage  method.  This 
method  consists  in  passing  through  the  coil  a  direct  current,  the 
value  of  which  is  noted  by  an  ammeter,  while  the  drop  of  voltage 
across  the  coil  is  measured  by  a  voltmeter.  Then  the  resistance  is 


ALTERNATING-CURRENT  MACHINERY 


311 


rr 

R=  — .    Knowing  the  resistance  of  the  coils,  we  can  determine  the 
drop  in  voltage  due  to  resistance,  under  load.    This  loss  of  electro- 


Transformer 

287.     Wiring  Diagram  for  Core  Loss  and  Exciting  Current  Test 

motive  force  is  usually  expressed  in  per  cent  of  the  electromotive 
force  supplied  to  the  primary. 

Impedance.  One  of  the  important  constants  of  a  transformer 
is  its  impedance  ratio,  that  is,  the  ratio  of  the  voltage  consumed  by 
its  total  internal  impedance  at  full-load  current  to  its  rated  full- 
load  voltage.  The  impedance  of  a  transformer  is  measured  by  short- 
circuiting  one  of  its  windings,  impressing  an  alternating  electromotive 
force  on  the  other  winding  and  making  simultaneous  measurements 
of  current  and  impressed  voltage.  E  and  I  being  thus  observed, 


7r  OL  nsformer 

Fig.  288.     Diagram  of  Connections  for  Making" Transformer  Impedance  Test 


the  impedance  is  found  as  Z=  —  ohms.    The  value  thus  obtained 
includes  the  impedance  of  both  primary  and  secondary  windings. 


312  ALTERNATING-CURRENT  MACHINERY 

Impedance  may  be  considered  as  constant  at  all  loads.  It  is 
usually  measured  at  full-load  current,  and  the  impressed  voltage 
is  then  called  the  impedance  volts,  and  when  expressed  in  per  cent  of 
the  rated  voltage  of  the  transformer,  it  is  called  the  per  cent  im- 
pedance drop.  It  is  evident  that  the  '  Impedance  ratio"  as  defined 
above  is  simply  the  per  cent  impedance  drop  divided  by  100. 

The  impedance  of  a  transformer  is  an  important  factor  in 
determining  the  regulation  by  calculation.  To  determine  the  im- 
pedance voltage,  the  transformer  is  connected  as  shown  in  Fig.  288. 
As  the  impedance  voltage  is  not  very  large  —  varying  from  2  per 
cent  to  6  per  cent  of  rated  primary  voltage  in  standard  transformers 
—  a  much  more  accurately  readable  deflection  of  the  voltmeter  will 
be  obtained  if  the  primary  coil  is  connected  to  the  mains.  As  will 
be  seen  by  referring  to  Fig.  288,  the  secondary  coil  is  short-circuited. 
The  primary  coil  is  connected  in  series  with  an  adjustable  resistance 
to  the  low-voltage  mains.  The  resistance  is  slowly  cut  out,  until 
full-load  current  flows  in  the  coil,  as  indicated  by  the  ammeter. 
Then  the  voltmeter  indicates  the  impedance  voltage.  This  should 
be  expressed  in  per  cent  of  the  normal  voltage  of  the  coil.  From 
the  equation 


the  total  impedance,  inductance,  and  reactance  can  be  computed, 
provided  R  and  the  frequency  /  are  known. 

Example.  In  a  test  on  a  7.5-kw.  transformer  with  secondary  short- 
circuited  and  primary  connected  to  2,080-volt  mains,  the  impedance  voltage 
was  61.1  volts  at  full-load  current  of  3.6  amperes  in  the  primary,  at  a  fre- 
quency of  60  cycles.  The  impedance  drop  being  61.1,  the  per  cent  impedance 

drop  is       Q     =2.935  per  cent,  and  the  impedance  is  Z=-^-  =  16.95  ohms. 


Since   impedance  =  Z  =  Vfl2+Z2,     X=^Z2-R2  and  the  total  reactive 
drop  expressed  in  per  cent  is 

%XI  =  V(%  impedance  drop)2-(%fl/)2 
If  %  #7  =  1.57  from  test,  and  %  ZI  =2.935  as  calculated  above,  then 


%XI  =  \  2.935    -  1.57     =  2.48 
or 

2.48       2080 


ALTERNATING-CURRENT  MACHINERY  313 

Regulation.  The  definition  of  regulation  given  on  page  266  will 
be  repeated  here,  it  is  :  the  rise  of  secondary  terminal  voltage  from  rated 
non-inductive  load  to  no  load  (at  constant  primary  impressed  terminal 
voltage)  expressed  in  per  cent  of  the  secondary  terminal  voltage  at  rated 
load.  The  regulation  of  a  transformer  may  be  determined  directly 
by  exciting  the  transformer  at  rated  frequency  and  with  a  primary 
voltage  such  that  rated  secondary  voltage  is  obtained  at  full  load, 
using  lamps*  or  a  water  rheostat  as  load.  The  increase  in  secondary 
terminal  voltage  from  full  load  to  no  load  is  then  observed,  the 
primary  voltage  and  frequency  being  kept  constant  throughout 
the  test.  This  increase  in  secondary  voltage  divided  by  the  second- 
ary full  load  voltage  is  then  the  regulation  expressed  in  per  cent, 
This  method  is,  however,  unsatisfactory,  because  of  the  small  differ- 
ence between  the  full  load  and  no  load  values,  and  the  liability  of 
error  in  measuring  either  of  them-.  Much  more  reliance  can  be  placed 
on  results  calculated  from  separate  measurements  of  impedance 
drop  and  resistance  than  on  actual  measurement  of  regulation. 

A  number  of  methods  have  been  proposed  for  the  calculation  of 
transformer  regulation,  but  the  following  formula  will  be  found 
simple  and  practically  correct. 


per  cent  regulation  =  pRI  +  qXI  +  ~^  —  -     (40)    . 

200 

in  which  RI  is  total  resistance  drop  in  the  transformer  due  to  load 
current  expressed  in  per  cent  of  rated  voltage;  XI  is  total  reactive 
drop  due  to  load  current  similarly  expressed;  p  is  power  factor  of  the 
load  on  the  secondary  =  cos  6;  for  a  non-inductive  load  p=l;  and 
q  is  reactive  factor  of  the  load  =  sin  0. 

Example.     A  7.5-kw.,  60-cycle  transformer  of  10  to  1  ratio  with  secondary 
voltage  of  208  at  full  load  has  the  following  constants  : 

Primary  resistance,  #1  =  5.65  ohms.  20  34 

Primary  #i/i  =  5.65  X  3.6=20.34  volts,  or  %RJi  =  —  :  —  X100  =0.978%. 
Secondary  resistance,  R2=  0.0334  ohm. 

Secondary  R2Iz  =  0.0334X36  =  1.24  volts,   or    %#2/2  =  0.596%. 
Total  RI  (reduced  to  primary)  =20.34  +  10  X  1.24  =32.74  volts,  or  1.57%. 
Reactive  drop,  XT'  =2.48%. 
(a)     Regulation  on  non-inductive  load. 
Here  cos  6  =l=p,  and  sin  0=  0  =  q. 

%  regulation  =  1  X  1.57  +  0  X  2.48  +  (*  X  2.48  -1.57  X  0)« 

200 
=  1.57  +  0.031  =  1.60 


314 


ALTERNATING-CURRENT  MACHINERY 


(b)     Regulation  on  inductive  load. 


Assuming  the  power  factor  of  the  load  to  be  0.80  =  p,  gives  q 


=  yi  -o. 


=0.6 


%  regulation  =  0.8  X  1.57  +  0.6  X  2.48  + 

=  1.256  +  1.488  +  0.0054  =  2.75. 


(0.8  X  2.48  -0.6  X1.57)2 


200 


The  regulation  for  any  other  power  factor  p  may  be  calculated  in  a  similar 
manner. 

Efficiency  Calculation.  The  efficiency  of  any  piece  of  apparatus 
at  a  given  load  is  equal  to  the  output  divided  by  the  input.  The 
input  is  equal  to  the  output  plus  the  losses.  The  efficiency  may  then 
be  defined  as  the  ratio  of  the  output  to  the  output  plus  the  losses.  In 
nearly  every  case  the  efficiency  can  be  determined  more  accurately 
by  measuring  the  losses,  and  then  computing  the  efficiency  accord- 


<    Su00ly  Mofn 

A 

0000, 

B 

nnnn  , 

row] 

<7|—l/OW/AS-*|/> 


0000 


Fig.  289.     Transformer  Connected 
for  Polarity  Test 


Fig.  290.     Wiring  Diagram  for  Transformer 
Polarity  Test  . 


ing  to  the  second  definition,  than  by  attempting  to  measure  the  total 
output  and  input,  and  then  taking  their  ratio. 

Example.  A  given  5-kw.  transformer  is  rated  at  2,000  volts  primary, 
and  200  volts  secondary,  at  a  frequency  of  60  cycles  per  second.  The  coil 
resistances  are  found  by  measurement  to  be: 

Primary  coil  resistance 10.1  ohms 

Secondary  coil  resistance 0.067  ohms 

At  full  load,  full-load  currents  are: 

Primary  current 2.5  amperes 

Secondary  current 25.0  amperes 

Core  loss,  as  determined  by  test 70  watts 

Copper  losses  at  full-load  are: 

Primary  loss  =I'2Rf  =  10.1  X  (2.5)2  = 63  watts 

Secondary  loss  =  I"2R"  =0.067  X  (25)2  = 42  watts 


ALTERNATING-CURRENT  MACHINERY  315 

Total  loss  at  full-load 175  watts 

Full-load  output 5,000  watts 

Full-load  intake 5,175  watts 

Full-load  efficiency  5,000  -=-5,175  = 96.6  per  cent 

At  half  load:      , 

Total  I2R  loss 26  watts 

Core  loss 70  watts 

Total  loss 96  watts 

Half-load  output 2,500  watts 

Half-load  intake 2,596  watts 

Half-load  efficiency,  2,500 -=-2,596  = 96.3  per  cent 

The  all-day  efficiency  of  a  transformer  is  the  ratio  of  the  output  of  work 
(watt-hours)  during  the  day  to  the  total  input  of  work  (watt-hours).     The  usual 
conditions  of  practice  will  be  met  if  the  calculation  is  based  upon  5  hours  at 
full  load,  and  19  hours  at  no  load.      See  page  265. 
Output : 

5  hours  at  full  load  =  5  hours  X  5,000  watts  =  25,000  watt-hours 
19  hours  at  zero  load  =  0  watt-hours 

Total  output  in  24  hours  =  25,000  watt-hours 

Input : 

5  hours  at  full  load  =    5hrs.  X  5,175  watts  =  25,875  watt-hours 
19  hours  at  zero  load  =49hrs.  X       70  watts  =    1,330  watt-hours 

The  zero  load  intake  is  but  very  little  more  than  core  loss,  since  I'2R'  is 
negligible  at  zero  load. 

Total  intake  in  24  hours  27,205  watt-hours 

All-day  efficiency  25,000  -=-27,205  =     91.9  per  cent 

Polarity  Test.  Transformers  are  generally  designed  so  that 
the  instantaneous  direction  of  flow  of  the  current  in  certain  selected 
leads  is  the  same  in  all  transformers  of  the  same  type.  For  example, 
the  transformer  shown  in  Fig.  289  is  designed  so  that  the  current 
at  any  instant  flows  into  lead  A  and  out  of  lead  C.  Such  transformers 
run  properly  in  parallel  when  similar  primary  and  secondary  leads 
on  different  transformers  are  connected  together.  The  primaries  of 
two  transformers  A  and  B  are  connected  to  supply  mains,  the  con- 
nections of  one  primary  being  made  without  reference  to  the  con- 
nections of  the  other.  It  is  desired  to  determine  first,  how  the  sec- 
ondaries are  to  be  connected  in  parallel  to  supply  current  to  one  and 
the  same  receiving  circuit;  and  second,  how  the  secondaries  are  to  be 
connected  in  series  to  supply  current  to  a  receiving  circuit.  This 
test  is  made  as  follows : 

Connect  a  terminal,  say  a,  Fig.  290,  of  the  secondary  of  one  transformer 

to  one  terminal  c  of  the  secondary  of  the  other  transformer.  Then  connect  two 
110-volt  lamps  in  series  (or  a  voltmeter)  to  the  other  two  terminals  6  and  d. 
If  the  lamps  do  not  light  (or  if  the  voltmeter  gives  no  deflection),  then  first, 


316  ALTERNATING-CURRENT  MACHINERY 

the  terminals  a  and  c  are  the  proper  ones  to  connect  together  to  one  service 
main;  and  the  terminals  b  and  d  are  to  be  connected  together  to  the  other  service 
main ;  and  second,  the  terminals  a  and  c  are  not  the  proper  ones  to  be  connected 
together,  but  6  and  c  are  properly  connected  together  in  order  to  connect 
the  two  coils  in  series,  the  other  two  terminals  a  and  d  being  connected  to  the 
service  mains. 


ALTERNATING-CURRENT 
MACHINERY 


PART  V 

CONVERSION  OF  ALTERNATING  INTO 
DIRECT  CURRENT 

In  spite  of  the  increasing  use  of  alternating  currents,  there  is 
always  a  demand  for  devices  of  various  kinds  for  converting  the 
power  received  from  an  alternating-current  circuit  into  power  in 
the  form  of  direct  current.  Thus,  many  electro-chemical  processes, 
electrolytic  refining  of  silver  and  copper,  the  charging  of  storage 
batteries,  and  direct-current  motors  all  require  direct  current.  Elec- 
tric power,  as  is  well  known,  can  be  transmitted  long  distances  most 
economically  by  the  three-phase  alternating-current  system  employ- 
ing high  voltages.  The  demand  for  direct-current  power  arises 
when  the  power  is  to  be  utilized  at  the  receiving  end  of  the  line. 

The  conversion  from  alternating  current  to  direct  current  is 
accomplished  in  practice  in  the  following  ways:  (a)  by  the  rectifying- 
commutator;  (b)  by  the  aluminum  valve  rectifier;  (c)  by  the  mercury- 
vapor  rectifier;  (d)  by  the  rotary  (or  synchronous)  converter;  (e)  by 
the  motor-generator. 

Rectifying  Commutator.  The  rectifying  commutator  is  a  com- 
mutator driven  at  a  speed  synchronous  with  the  alternating  current 
supplied  to  its  brushes  and  it  reverses  the  connections  of  the  armature 
windings  of  an  alternator  as  a  whole.  The  rectifying  action  consists 
in  reversing  alternate  (negative)  half  waves  of  the  current  delivered 
by  the  alternator  so  that  the  current  in  the  receiving  circuit  is  always 
in  one  direction.  The  application  of  this  device  to  the  compounding 
of  alternators  is  explained  on  page  111. 

The  rectifying  commutator  is  limited  to  the  rectifying  of  com- 
paratively small  currents  at  moderate  voltages  on  account  of  the 
prohibitive  sparking  which  occurs  at  the  brushes  at  high  voltages 

and  large  outputs. 

« 

Copyright  1912,  by  American  School  of  Correspondence. 


318 


ALTERNATING-CURRENT  MACHINERY 


Aluminum  Valve  Rectifier.  The  aluminum  valve  rectifier  is 
an  electrolytic  coil  which  depends  on  the  property  of  aluminum 
electrodes  in  certain  electrolytes  to  let  electric  current  pass  in  one 
direction  (when  the  aluminum  electrode  is  cathode)  but  not  in  the 
other.  The  simplest  arrangement  of  a  single- cell  valve  rectifier 
is  to  connect  the  secondary  of  the  transformer  whose  current  is  to  be 
converted  into  direct  current  to  the  positive  plate  (iron)  of  the  cell 
and  the  negative  plate  (aluminum)  to  the  positive  terminal  of  the 
storage  battery  to  be  charged.  To  complete  the  series  circuit  the 
negative  terminal  of  the  storage  battery  is  connected  to  the  other 
end  of  the  transformer  secondary.  A  common  glass  battery  jar, 
containing  an  approximately  neutral  solution  of  pure  ammonium 
phosphate  in  which  an  iron  plate  (the  positive)  and  an  aluminum 


/OAflF 


Fig.  291.     Diagram  of  Connections  for  Aluminum  Valve  Rectifier 

plate  (the  negative)  are  immersed,  constitutes  the  rectifier.  With 
such  a  single  cell  only  one-half  of  the  complete  alternating-current 
wave  is  utilized,  the  other  half  of  the  current  wave  of  reversed  sign 
being  suppressed  by  the  aluminum  valve  action.  The  reversed  cur- 
rent which  would  tend  to  flow  from  the  aluminum  plate  through  the 
electrolyte  to  the  iron  plate  is  stopped  because  of  the  formation  of 
a  thin  insulating  film  of  aluminum  oxide  over  the  aluminum  plate 
which  resists  the  passage  of  current  unless  the  voltage  exceeds  about 
150  volts. 

A  better  plan  of  connecting  a  single  cell  so  that  both  halves 
of  the  alternating-current  wave  will  be  rectified  is  given  in  Fig.  291. 
The  choke  coil  C  is  an  autotransformer,  tapped  at  its  middle  point 
by  a  connection  to  the  direct-current  load  B,  such  as  a  storage 


ALTERNATING-CURRENT  MACHINERY          319 

battery  to  be  charged.  The  other  terminal  of  the  battery  is  connected 
to  the  iron  plate  in  the  cell.  The  coil  C  offers  a  high  impedance  to 
the  alternating  current,  but  very  little  impedance  to  the  intermittent 
direct  current  flowing  out  from  its  middle  tap,  because  an  equal  amount 
of  current  comes  from  each  end  and  flows  in  opposite  directions  around 
the  core  of  the  choke  coil.  Assuming  that  at  a  given  instant  the 
terminal  A  of  the  secondary  of  the  supply  transformer  is  positive, 
the  path  of  the  current  will  be  as  shown  by  the  arrowheads.  Since 
current  cannot  enter  the  solution  of  the  cell  by  way  of  an  aluminum 
electrode,  the  5  amperes  have  to  pass  through  one-half  of  the  choke- 
coil  winding  and  out  of  the  middle  tap  into  the  battery  B,  from  which 
it  enters  the  rectifier  cell  through  the  iron  electrode  and  returns 
through  the  aluminum  electrode  at  the  right,  back  to  the  other  ter- 
minal D  of  the  transformer  secondary.  Since  the  tap  point  of  the 
autotransformer  C  is  at  its  middle  point,  the  energy  delivered  to  its 
lower  half  acting  as  a  primary  coil  is  transferred  by  transformer  action 
to  its  upper  half,  so  that  the  10-ampere  pulsating  direct  current 
is  furnished  by  the  combined  currents  of  5  amperes  in  each  of  the 
two  halves  of  the  coil  C.  It  follows  that  at  any  instant  there  is  twice  as 
much  current  through  the  direct-current  circuit  B  as  through  the 
alternating-current  circuit,  but  at  half  the  voltage. 

Thus,  if  both  the  alternating-current  and  the  direct-current 
voltages  are  measured  by  alternating-current  voltmeters,  which  give 
effective  values,  the  alternating-current  voltage  would  be,  say,  100 
volts,  and  the  direct-current  voltage  would  be  50  volts.  In  prac- 
tice the  direct-current  voltage  would  be  less  than  50  volts  on  ac- 
count of  voltage  drops  and  leakage  in  the  cell. 

With  each  reversal  of  the  alternating-current  voltage,  the  direc- 
tion of  the  current  in  the  alternating-current  circuit,  including  the 
coil  C,  is  reversed  and  each  aluminum  electrode  alternates  with  the 
other  in  becoming  an  active  negative  terminal  and  then  an  inactive 
positive,  but  the  direction  of  flow  of  the  direct  current  through  the 
battery  B  will  remain  unchanged. 

The  aluminum  valve  rectifier  is  adapted  for  rectifying  relatively 
small  currents  (up  to  about  25  amperes  direct  current),  its  capacity 
being  limited  by  the  heating  of  the  cells.  The  efficiency  is  low,  rang- 
ing from  50  to  60  per  cent  in  practice.  The  power  factor  is  never 
over  90  per  cent,  even  on  full  load.  The  highest  effective  value  of 


320  ALTERNATING-CURRENT  MACHINERY 

the  alternating-current  voltage  that  can  be  used  with  the  cell  at 
normal  operating  temperatures  is  about  175  volts,  which  means 
that  about  55  volts  will  be  obtained  on  the  direct-current  side. 

As  electrolytes,  ammonium  phosphate  or  sodium  phosphate 
are  considered  best.  The  cell  should  be  artificially  cooled  to  work 
satisfactorily.  The  voltage  may  be  regulated  by  means  of  resistance 
in  either  the  alternating-current  or  the  direct-current  circuits,  but 
a  better  method  is  by  adjusting  the  alternating-current  voltage 
by  a  small  autotransformer  having  a  number  of  taps.  The  greatest 
advantage  of  the  aluminum  valve  rectifier  is  its  cheapness  and  sim- 


I 


A 


lv 

B 


Fig.  292.     Mercury- Vapor  Arc  Rectifiers 

plicity.  On  account  of  its  small  direct-current  output,  its  poor 
efficiency,  and  comparatively  high  cost  of  maintenance,  it  is  not  as 
yet  of  great  commercial  importance. 

Mercury=Vapor  Arc  Rectifier.  The  mercury-vapor  arc  rectifier, 
like  the  aluminum  valve  rectifier,  operates  by  the  action  of  electric 
valves.  The  rectifier  bulb  consists  of  a  closed  glass  vessel  provided 
with  four  electrodes;  those  marked  A  A,  Fig.  292,  called  anodes  (or 
positives)  are  of  graphite,  and  the  other  two  B  and  (7,  called  cathodes, 
are  of  mercury.  The  air  is  exhausted  from  the  bulb  which  contains 
only  mercury  vapor.  This  like  other  metal  vapors  is  an  electrical 


ALTERNATING-CURRENT  MACHINERY 


321 


i — MAMMA/ — i 

<4.C.  SUPPLY 


conductor  under  some  conditions,  and  the  graphite  or  positive 
electrodes  are  immersed  in  this  vapor.  A  pool  of  mercury  in  the 
bottom  of  the  bulb  forms  the  negative  electrode  or  cathode  By 
and  the  small  electrode  C  is  used  merely  for  starting  the  mercury  arc 
between  A  and  B. 

The  rectifying  action  depends  on  the  properties  of  mercury  in 
the  presence  of  mercury  vapor,  as  follows:  Current  can  readily^ 
pass  from  either  of  the  solid  graphite  electrodes  to  the  mercury 
vapor,  but  if  it  is  attempted  , 

to  reverse  the  direction  of  the 
current,  a  very  high  apparent 
resistance  is  developed  at  the 
surface  of  the  mercury  elec- 
trode which  prevents  the  flow 
of  current  from  B  to  A.  The 
diagram  of  electrical  connec- 
tions for  a  mercury  rectifier  is 
given  in  Fig.  293.  The  alter- 
nating-current supply  circuit 
is  connected  to  the  two  posi- 
tive electrodes  A  and  A'  of 
the  bulb  and  also  to  two  re- 
actance coils  F  and  E.  On 
account  of  the  check-valve 
action  just  described,  the  pul- 
sations of  the  alternating  cur- 
rent pass  alternately  from  A 
and  A'  into  the  mercury  (neg-  Fig.  293. 
ative)  electrode  B,  from  which 
they  are  delivered  as  an  uni-directional  current  for  charging  the 
storage  battery  J.  When  the  bulb  starts  to  rectify  there  is  a  high 
resistance  at  the  surface  of  the  mercury  which  must  be  broken  down 
before  current  can  pass.  This  cathode  resistance  acts  like  an  insulat- 
ing film  over  the  surface  of  the  mercury,  but  wheii  a  current  is  once 
started  it  will  continue  to  flow,  meeting  with  but  small  resistance 
as  long  as  the  current  is  uninterrupted.  The  briefest  interruption, 
however,  permits  the  cathode  resistance  to  increase  enormously, 
and  thus  stop  the  action  of  the  bulb. 


Connections  for  Mercury- Vapor 
Arc  Rectifier 


322  ALTERNATING-CURRENT  MACHINERY 

In  order  to  break  down  the  cathode  resistance,  on  starting  the 
rectifier,  the  bulb  is  tilted  or  shaken  so  that  the  small  starting  anode 
C,  Fig.  293,  is  brought  into  contact  with  the  cathode  B  by  a  mercury 
bridge.  Current  then  passes  between  C  and  B  and  the  little  stream 
of  mercury  which  bridges  the  space  between  the  electrodes  breaks 
with  a  spark  "when  the  bulb  is  returned  to  its  normal  vertical  posi- 
tion. This  spark  or  initial  arc  breaks  down  the  cathode  resistance 
by  forming  mercury  vapor  which  enables  the  graphite  anodes  to 
become  active  and  the  rectifying  action  to  start.  After  the  tube  is 
in  operation,  the  circuit  through  C  is  opened  by  a  switch. 

The  action  of  the  rectifier  may  be  followed  in  detail  with  the 
aid  of  Fig.  293.  Let  us  assume  an  instant  when  the  terminal  // 
of  the  supply  transformer  is  positive,  then  the  path  of  the  current 
will  be  shown  by  the  small  arrowheads,  the  electrode  A  will  be 
positive,  and  the  current  is  free  to  flow  from  A  across  the  mercury 
vapor  to  By  the  mercury  negative  electrode  (cathode).  From  B 
the  current  passes  through  the  storage  battery  J,  through  the  re- 
actance coil  E,  and  back  to  the  negative  terminal  G  of  the  trans- 
former. A  moment  later  when  the  impressed  voltage  falls  below  a 
value  sufficient  to  maintain  the  arc  against  the  counter-electromotive 
force  of  the  arc  and  storage  battery,  the  current  would  be  interrupted 
at  the  end  of  the  first  half  cycle  before  the  current  from  anode  A' 
could  be  established. 

In  order  to  prevent  this  interruption  two  reactance  (choke) 
coils  F  and  E  are  connected  in  the  circuit,  as  shown.  The  inductance 
of  these  coils  delays  the  decreasing  current  from  one  anode,  as  A,  until 
the  voltage  of  the  transformer  has  passed  through  zero,  reverses, 
and  builds  up  to  such  a  value  as  to  cause  A'  to  start  an  arc  between 
it  and  the  mercury  cathode  B.  The  path  of  the  current  is  now  from 
G  down  to  A'y  across  the  mercury  vapor  to  B,  out  through  J,  the 
coil  F,  and  back  to  H,  which  is  now  the  negative  terminal.  The  new 
path  is  indicated  by  the  arrows  enclosed  in  circles.  A  moment 
later  A  again  becomes  active,  and  the  path  of  the  current  is  again 
indicated  by  the  plain  arrowheads  as  at  first.  Mercury-vapor 
rectifiers  as  made  for  charging  storage  batteries,  operating  arc  lamps, 
small  direct-current  motors,  etc.,  are  furnished  in  four  sizes,  suitable 
for  10,  20,  30,  and  40  amperes  of  direct  current,  respectively.  They 
are  designed  for  a  frequency  of  60  cycles,  but  can  be  adapted  to  com- 


ALTERNATING-CURRENT  MACHINERY  323 

mercial  frequencies  from  25  to  140  cycles  per  second.  They  are  made 
for  the  standard  secondary  alternating  voltages  of  110  and  220  volts. 
The  efficiency  varies  with  the  direct-current  voltage  delivered, 
and  ranges  from  about  75  per  cent  from  one  quarter  to  full  load  when 
the  direct-current  voltage  averaged  80  volts,  up  to  over  80  per  cent 
for  direct-current  voltages  averaging  112  volts. 


Fig.  294.     Front  and  Back  View  of  Mercury- Vapor  Arc  Rectifier  Equipment 

The  rectifier  tubes  differ  in  size  according  to  their  ampere  rating 
and  in  shape  according  to  the  direct-current  voltage  for  which  they 
are  designed.  The  average  life  of  a  tube  under  normal  operating 
conditions  is  about  600  hours,  when  a  new  tube  should  be  substituted. 

Fig.  292  shows  two  different  voltage  types  of  the  30-ampere 
size  of  tube  made  by  the  General  Electric  Company.  The  tube  on  the 
left  is  for  direct-current  voltages  from  25  to  45  volts;  and  the  tube 
on  the  right  is  for  90  to  250  volts. 

A  wide  range  of  variation  in  the  direct-current  voltage  is  easily 
obtained  by  manipulating  the  dial  switch  mounted  on  the  panel, 
shown  in  Figs.  294  and  295, 


324 


ALTERNATING-CURRENT  MACHINERY 


Fig.  294  shows  front  and  rear  views  of  a  complete  mercury  arc 
rectifier  outfit  with  switchboard  panel,  as  built  by  the  General 
Electric  Company.  Fig.  295  shows  the  front,  rear,  and  side  views 
of  such  an  outfit  with  standard  dimensions.  The  outfit  consists  of 
a  rectifier  tube,  reactance,  regulating  compensator,  and  panel. 
The  panel  is  completely  wired  and  simply  requires  connections  to  be 
made  to  the  secondaries  of  the  supply  transformer  and  to  the  direct- 
current  load  circuit.  The  equipment  of  the  panel  is  clearly  shown 
in  Fig.  295. 

The  double-pole  "line  switch"  to  the  left  in  Figs.  294  and  295 
connects  to  the  alternating  supply  circuit.  The  circuit  breaker  at  the 


LO40 


C/RCU/T  B/t£AHER 
AMMETER 
•RECT/F/ER  SHAKER 
ftODE  SMTCH 
STAftT/MG  -SW/TCH 


FUSE 


REGULAT/M6 
COMFEMSA7VR 


ftEACTAMCE 


Fig.  295.     Diagrammatic  Views  of  Front,  Back,  and  Side  of  Rectifier  Equipment 
Showing  Details 

top  of  the  panel  protects  the  direct-current  circuit  from  excessive 
current.  The  "starting  switch"  is  a  single-pole  double-throw 
spring  switch  which  is  used  to  start  the  tube  on  a  resistance  mounted 
on  the  back  of  the  panel.  The  "anode  switch"  is  an  auxiliary  spring 
switch  mechanically  operated  by  the  "starting  switch"  and  auto- 
matically opens  the  starting  anode  circuit  as  soon  as  the  handle  of 
the  "starting  switch"  is  released. 

The  dial  switch  is  provided  with  a  double  set  of  contact  buttons, 
one  set  for  rough  and  the  other  for  fine  regulation  of  the  direct- 
current  voltage.  These  contact  buttons  are  connected  to  taps 


ALTERNATING-CURRENT  MACHINERY  325 

brought  out  from  the  regulating  compensator  mounted  at  the  back 
of  the  panel. 

Another  important  application  of  the  mercury-vapor  rectifier 
is  in  connection  with  street  and  out-door  lighting  by  series,  constant, 
direct-current  arc  and  incandescent  lamps.  The  arc  lamps  commonly 
used  for  this  service  are  known  as  "luminous  arc"  and  "flame  arc" 
lamps.  The  series  incandescent  lamps  for  this  service  have  filaments  - 
of  tungsten,  and  are  made  for  candle  powers  ranging  from  25  to  80, 
and  for  currents  of  4  and  6.6  amperes. 

This  system  of  series,  constant,  direct-current  lighting  involves 
an  alternator  delivering  alternating  current  at  2,200  volts  to  the 
primary  of  a  constant-current  transformer  whose  secondary  is 
designed  to  give  a  constant  alternating  current  of  either  4  or  6.6 
amperes  at  a  voltage  depending  upon  the  number  of  arc  lamps  in 
series  to  be  supplied.  A  mercury-vapor  rectifier  is  then  used  to 
convert  the  constant  alternating  current  into  a  constant  direct 
current  suitable  for  use  in  the  arc  lamps. 

The  rectifiers  constructed  for  this  service  differ  from  those 
described  above  only  in  certain  details.  The  rectifier  tubes  are 
operated  in  oil-filled  tanks,  for  cooling  purposes,  and  the  voltages 
of  the  rectified  current  range  from  1,200  volts  in  the  12-light  outfit 
up  to  6,450  volts  in  the  75-light  outfit.  Rectifier  tubes  can  be  built 
for  voltages  up  to  13,000. 

ROTARY  OR  SYNCHRONOUS  CONVERTER 

The  rotary  or  synchronous  converter  is  a  machine  for  convert- 
ing alternating  current  into  direct  current,  or  vice  versa.  The  import- 
ance that  such  machine's  have  assumed  in  the  electrical  industry 
is  due  to  several  causes: 

(a)  It  is  necessary,  for  economic  reasons,  to  use  alternating 
current  at  high  voltages  in  long-distance  transmission,  as  explained 
on  page  4.      Therefore,  rotary   converters  are  required  for  chang- 
ing the  alternating  current  into  direct  current  for  use  in  electric 
railway  motors,  which  must  be  supplied  with   direct  current  from 
the  trolley  wire  at  points  at  a  distance  from  the  power  house. 

(b)  Rotary  converters  are  needed  for  charging  storage  bat- 
teries in  places  where  the  central  station  supplies  alternating  cur- 
rent, and  inverted  rotaries  are  necessary  for  factory  driving  with 


326  ALTERNATING-CURRENT  MACHINERY 

alternating-current  motors  in  cases  where  direct  current  only  is 
supplied  by  central  stations. 

(c)  Direct  current  is  necessary  in  many  of  the  chemical  and 
electro-metallurgical  industries  such  as  the  electrolytic  reduction  of 
aluminum  from  its  ores,  the  electrolytic  refining  of  copper,  etc.  If 
alternating  current  is  generated  and  transmitted  to  these  establish- 
ments, it  must  be  converted  into  direct  current  before  it  can  be 
utilized. 

The  rotary  converter  is  chiefly  used  to  convert  polyphase  alter- 
nating currents  into  direct  current  on  a  large  scale. 

Comparison  with  Direct=Current  Dynamo.  In  general  appear- 
ance and  construction,  the  rotaiy  converter  resembles  the  direct- 
current  generator  very  closely.  The  chief  outward  difference  is  the 
addition  of  a  number  of  collector  rings  concentric  with  the  shaft  on 
one  side  of  the  armature,  and  the  commutator  is  very  much  larger 
than  in  the  ordinary  direct-current  generator.  Another  point  of 
difference  is  in  the  relative  dimensions  of  the  magnetic  circuit, 
including  yoke  and  magnet  cores,  which  are  smaller  than  would  be 
usual  or  desirable  in  ordinary  direct-current  generators. 

Under  the  usual  condition  of  running,  the  armature  is  driven, 
as  in  a  simple  synchronous  motor,  by  alternating  current  supplied 
to  the  collector  rings  from  an  external  source.  While  so  revolving 
direct  current  can  be  taken  from  brushes  bearing  upon  the  com- 
mutator. 

The  current  in  the  armature  of  a  rotary  converter  may  be 
thought  of  as  the  difference  between  the  inflowing  alternating  cur- 
rents and  the  outflowing  direct  current.  The  average  value  of  the 
current  in  a  given  armature  conductor  is,  therefore,  smaller  in  value 
than  in  the  corresponding  direct-current  generator,  and  the  heat- 
ing effect  PR  is  correspondingly  less'.  Furthermore,  the  magnetizing 
action  of  the  inflowing  alternating  current  upon  the  armature  is 
almost  completely  neutralized  by  the  magnetizing  action  of  the 
outflowing  direct  current.  Therefore,  a  larger  number  of  smaller 
conductors  may  be  wound  upon  a  given  armature  core  if  the  arma- 
ture is  to  be  used  for  a  rotary  converter,  than  would  be  permissible 
if  the  armature  were  to  be  used  for  a  direct-current  generator. 
That  is,  the  allowable  power  output  of  a  machine  of  given  size 
is  not  limited  to  so  small  a  value  if  the  machine  is  to  be  used  as 


ALTERNATING-CURRENT  MACHINERY 


327 


TABLE  IX 
Power  Ratings  of  Rotary  Converters  in  Kilowatts 


Continuous- 

Single- 

Three- 

Four- 

Six- 

Current 

Phase 

Phase 

Ring 

Phase 

Dynamo 

Converter 

Converter 

Converter 

Converter 

100 

85 

132 

162 

192 

/ f+Brush 


a  polyphase  rotary  converter,  as  it  would  be  if  the  machine  were 
to  be  used  as  a  direct-current  generator. 

Table  IX  gives  the  power  ratings  which  a  machine  that  would 
be  rated  at  100  kw.  if  used  as  a  direct-current  generator  has,  when 
it  is  used  as  a  single-phase,  three-phase,  two-phase  (four-ring),  and 
six-phase  converter,  respectively. 

To  Make  a  Direct=Current  Dynamo  into  a  Rotary  Converter. 
Consider  an  ordinary  bipolar*  direct-current  dynamo.  Imagine 
two  opposite  commutator  bars  of  the 
machine  to  be  marked  a  and  b,  re- 
spectively, Fig.  296.  Let  the  field  mag- 
net of  the  machine  be  excited,  and 
the  armature  be  driven  at  a  speed  of 
n  revolutions  per  second  in  a  counter- 
clockwise direction,  as  shown  by  the 
arrow.  At  a  given  instant  the  marked 
bars  will  be  midway  between  the 
direct-current  brushes,  as  shown  in  the 
figure.  Let  us  call  the  position  of  the 
armature  at  this  instant  the  position 
A,  Fig.  297,  and  let  us  consider  the 
way  in  which  the  electromotive  force 
between  the  given  pair  of  commutator  bars  a  and  b  changes  as 
the  armature  rotates. 

(1)  While  the  armature  is  making  the  first  quarter  of  a  revo- 
lution from  the  A  position,  the  bars  will  move  until  bar  a  touches 
the  +  brush  and  bar  b  touches  the  —  brush;  and  the  electromotive 
force  between  the  bars  will  grow  from  zero  to  the  full  value  E  of  the 
direct  electromotive  force  between  the  brushes. 


77- 


Fi; 


.  296.     Commutator  and  Brush 
)iagram  for  D.  C.  Dynamo  as 
a  Rotary  Converter 


*The  following  discussion  applies  to  multipolar  machines  also,  but  the  sta-tements  are 
much  simpler  when  limited  to  the  bipolar  machine. 


328 


ALTERNATING-CURRENT  MACHINERY 


(2)  While  the  armature  is  making  the  second  quarter  of  a 
revolution  from  the  A  position,  the  bars  will  move  until  they  are 
again  midway  beween  the  direct-current  brushes;  and  the  electro- 
motive force  between  the  bars  will  drop  from  the  value  E  to  zero. 

(3)  While  the  armature  is  making  the  third  quarter  of  a 
revolution  from   the  A  position,  the  bars  will   move   until  bar  a 
touches  the  —  brush,  and  bar  b  touches  the  +  brush;  and  the  electro- 
motive force  between  the  bars,  which  must  now  be  considered  as 
negative,  will  grow  from  zero  to  the  value  E. 

(4)  While  the  armature   is  making  the  fourth  quarter  of  a 
revolution  from  the  A  position,  the  bars  will  move  until  they  are 
again  midway  between  the  direct-current  brushes;  and  the  electro- 
motive force  between  the  bars,  which  is  still  to  be  considered  as 
negative,  will  drop  from  the  value  E  to  zero. 


first 


%ftevolufiort%fte\/ol<jtion  Devolution  ^Revo)utioj( 


Second 


Third     \     Fourth   i 


Position 


Fig.  297.     E.M.F.  Curve  for  One  Revolution  of  D.  C.  Dynamo 

These  successive  changes  of  electromotive  force,  between  the 
given  pair  a  b  of  commutator  bars,  which  occur  during  one  com- 
plete revolution  of  the  armature,  are  shown  graphically  in  Fig.  297. 

It  is  at  once  evident  that  the  electromotive  force  between  the 
bars  a  and  b  is  an  alternating  electromotive  force,  and  that  this 
alternating  electromotive  force  passes  through  a  cycle  of  values 
during  each  revolution  of  the  armature,  so  that  its  frequency  is  equal 
to  the  revolutions  per  second  of  the  armature  in  the  case  of  a  bipolar 
machine. 

Furthermore,  it  is  clear  that  the  alternating  electromotive  force 
between  a  given  pair  of  commutator  bars  a  b  on  a  direct-current 
dynamo  may  be  utilized  for  the  production  of  alternating  current; 
or  the  direct-current  dynamo  may  be  made  into  an  alternator  by 
providing  a  pair  of  insulated  metal  collecting  rings  connected  per- 
manently to  the  bars  a  and  6,  respectively,  and  which  are  kept  in 


ALTERNATING-CURRENT  MACHINERY  329 


/    / 
_  Z-/^ 


continuous  connection  with  an  outside  circuit  by  means  of  an 
auxiliary  pair  of  brushes,  that  is,  brushes  entirely  separate  and 
distinct  from  the  direct-current  brushes 
before  mentioned. 

A  direct-current  dynamo  made  into 
an  alternator  in  this  manner  but  with  its 
direct-current  brushes  and  commutator 

' 

kept  intact,  and  provided  with  two  col-  /           /         o0 

lecting  rings  only,  is  called  a  single-phase  \    "v     \  ^ 

rotary  converter.    When  the  machine  is  pro-  \   \x       °    \ 

vided,  as  explained  below,  with  three  col-  >:„  x"~  —  ~ 

lecting    rings,    four    collecting    rings,    or  y~7 

six  collecting  rings,  it  is  called  a  polyphase  f  / 

rotary  converter.  Fig.  298.   Brush  and  Commu- 


Three=Ring  Converter.     Three  equi- 


distant   commutator   bars    a,    b,    and   c, 

Fig.  298,  of  a  direct-current  dynamo  are  connected  to  three 
collector  rings.  It  is  shown  later  in  this  discussion  that  the 
electromotive  force  between  bars  a  and  b  is,  in  phase,  120  degrees 
ahead  of  the  electromotive  force  between  bars  b  and  c,  and  240  degrees 
ahead  of  the  electromotive  force  between  bars  c  and  a.  Three 
electromotive  forces  related  in  this  way  are  called  three-phase  electro- 
motive forces;  and  a  direct-current  dynamo 
provided  with  three  slip-rings  as  specified, 
is  called  a  three-phase,  or  three-ring,  rotary 
converter.  ,'' 

Four=Rmg    Converter.       Four     equi-       /'  /' 
distant    commutator    bars    a,    b,    a',    b',      !!  (  ;, 

Fig.  299,  of  a  direct-current  dynamo   are  °  } 

connected  to  four  collecting  rings.     Then       \    '*  ' 

the  electromotive  force  between  the  rings 
a  and  a'  is  at  its  zero  value  when  the 
electromotive  force  between  rings  b  and 
b'  is  at  its  greatest  value,  or  vice  versa.  Two 

Fig.  299.      Brush  and  Conimu- 

electromotive  forces  related  in  this  way  are          tator  Diagram  for  Four- 
Ring  Converter 

called  two-phase  electromotive  forces;  and  a 

direct-current    dynamo    with    four    collecting   rings,   as    specified, 

is  called  a  two-phase,  or  more  accurately,  a  four-ring  rotary  converter. 


330  ALTERNATING-CURRENT  MACHINERY 

Six=Ring  Converter.  Six  equidistant  commutator  bars  a,  b, 
c,  a',  b',  c',  Fig.  300,  of  a  direct-current  dynamo,  are  connected  to  six 
slip-rings.  Such  a  machine  is  called  a  six-phase  or  six-ring  rotary 
converter.  The  electromotive  force  between  the  rings  a  and  a'  is, 
in  phase,  120  degrees  ahead  of  the  electromotive  force  between  rings 
b  and  b',  and  240  degrees  ahead  of  the  electromotive  force  between 
rings  c  and  c'.  Three  electromotive  forces  so  related  are  called  three- 
phase  electromotive  forces;  and  a  six-ring  rotary  converter  may,  under 
certain  conditions,  be  supplied  with  three-phase  alternating  currents. 

Multipolar  Rotary  Converters.  The  discussion  already  given 
on  converters  refers,  for  the  sake  of  simplicity,  to  a  bipolar  machine. 
In  case  of  a  multipolar  d.  c.  dynamo,  having  p  field  poles,  the 
connections  of  the  n  rings  of  an  n-ring  converter  are  as  follows: 

7j 

Ring  1  is  connected  to  the  -•  -  equidistant  commutator  bars 

2i 

which,  for  a  given  position  of  the  armature,   are  squarely  opposite 

the  centers  of,  say,  the  north  poles  of 
the  field  magnet.  Let  d  be  the  distance 
between  the  commutator  bars  to  which 
ring  1  is  connected.  Ring  2  is  connected 

np 

to   the  —  equidistant   commutator  bars 

2      1 

which  are   -  -  of  d  ahead  of  the  bars  con- 
n 

nected   to   ring  1;  ring  3  is  connected  to 

np 

the  —  equidistant  commutator  bars  which 

2 

Fig.  300.    Brush  and  Commu-      are  -  -  of  d  ahead  of  the  bars  connected 

tator  Diagram  for  Six-  ^ 

Ring  Converter 

to  ring  1;  and  so  on. 

For  example,  a  6-pole  direct-current  dynamo  with  72  commu- 
tator bars,  to  be  made  into  a  three-ring  converter,  would  have  ring 
1  connected  to  commutator  bars  1,  25,  and  J$;  ring  2  connected  to 
commutator  bars  9,  33,  and  57;  and  ring  3  connected  to  commutator 
bars  17,  41,  and  66. 

E.  M.  F.  Relations  for  Rotary  Converter.  Let  E  be  the  value 
of  the  steady  electromotive  force  between  the  direct-current  brushes 
of  a  rotary  converter;  and  let  E2,  E9,  E*,  E6,  be  the  effective  values 


ALTERNATING-CURRENT  MACHINERY 


331 


of  the  alternating  electromotive  force  between  adjacent  collecting 
rings  of  a  two-ring,  three-ring,  four-ring,  and  six-ring  rotary  con- 
verter, respectively.  It  is  desired  to  find  the  relationship  between 
these  various  electromotive  forces. 

Relationship  between  E  and  E2.  The  maximum  value  of  the 
alternating  electromotive  force  between  the  collecting  rings  of  a 
two-ring  converter  occurs  at  the  instant  when  the  commutator 
bars  to  which  the  collecting  rings  are  connected  are  in  contact  with 
the  direct-current  brushes,  and  this  maximum  value  is,  of  course, 
equal  to  E.  Therefore,  the  effective  value  E2  of  the  alternating 
electromotive  force  between  the  slip-rings  of  a  two-ring  converter  is 

E 
equal  to    /— -.     That  is 


E 


(41) 


Relationship  between  E2  and  E3.  The  discussion  of  the  relation- 
ship between  E2  and  Es  will  be  carried  out  for  a  very  special  case, 
namely,  where  the  armature  has  18 
conductors;  and  this  special  discus- 
sion will  lead  to  a  very  simple  geo- 
metrical construction  which  will 
easily  give  the  relationship  between 
E2,  Es,  E*,  and  E6,  and  will  also 
show  the  phase  relations  of  the  vari- 
ous electromotive  forces  of  a  three- 
ring,  of  a  four-ring,  or  of  a  six-ring 
converter. 

Fig.  301  represents  a  direct- 
current  ring-wound  armature  having 
18  conductors,  each  conductor  representing  a  turn  of  wire.  Con- 
sider the  conductor  a.  This  conductor  has  an  alternating  electro- 
motive force  induced  in  it  as  the  armature  rotates.  Let  this  alter- 
nating electromotive  force  (effective  value)  be  represented  by  the 
short  line  a  in  Fig.  302. 

Consider  the  next  following  conductor  b.   The  alternating  electro- 
motive force  induced  in  this  conductor  has  the  same  value  as  that 


Fig.  301.     Diagram  of  a  D.  C.  Ring- 
Wound  Armature 


332 


ALTERNATING-CURRENT  MACHINERY 


induced  in  conductor  a,  but  is  behind  it  in  phase  by  the  angle 


Fig.  302.     E.M.F. 
Diagram  for 


360° 
18 

20°,  where  18  is  the  total  number  of  armature  conductors.  Let 
the  electromotive  force  induced  in  conductor  b  be 
represented  by  the  short  line  b  in  Fig.  302. 

Similarly,  the  short  lines  c,  d,  e,  /,  g,  etc.,  in  Fig. 
302,  represent  the  alternating  electromotive  forces 
(effective  values)  induced  in  the  conductors  c,  d,  e,  /, 
g,  etc. 

Consider  first  the  two-ring  converter.  Suppose 
that  its  slip-ring  1  is  connected  to  the  commutator  bar  which  is 
between  conductors  r  and  a,  as  shown  in  Fig.  301;  then  its  other 
slip-ring  will  be  connected  to  the  bar  which  is  between  conductors 
i  and  j,  and  the  alternating  electromotive  force  E2  between  these 
two  slip-rings  will  be  the  vector  sum  of  the  electromotive  forces  a, 
b,  c,  d,  e,  /,  g,  h,  and  i  (Fig.  302),  as  shown  in  Fig.  303.  That  is, 
E2  (effective  value)  is  represented  by  the  diameter  of  the  polygon 
(circle)  in  Fig.  303. 

Consider  now  the  three-ring  converter.  Suppose  its  three  rings 
are  connected  as  shown  by  the  numbers  1,  2,  and  3  in  Fig.  301. 
Then  the  electromotive  force  E's,  between  rings  1  and  2  is  the  vector 
sum  of  the  electromotive  forces  a,  6,  c,  d,  e,  and  /,  as  shown  in  Fig. 
303;  the  electromotive  force  E"a,  between  rings  2  and  3,  is  the  vector 

sum  of  the  electromotive  forces  g,  h,  i, 
j,  k,  and  /,  as  shown  in  Fig.  303  and 
the  electromotive  force  Er"a,  between 
rings  3  and  1,  is  the  vector  sum  of  the 
electromotive  forces  m,  n,  o,  p,  q,  and  r, 
as  shown  in  Fig.  303. 

Therefore,  the  effective  value  of  the 
electromotive  force  E%  between  the  two 
rings  of  a  two-ring  converter,  being 
represented  by  the  diameter  of  a  circle, 
the  effective  value  of  the  electromotive 
force  Eat  between  any  two  rings  of  a  three-ring  converter  is  repre- 
sented by  a  120-degree  chord  of  the  same  circle.  Therefore 


Fig.  303.     E.M.F.    Relations 
Two-  and  Three-Ring 
Converter 


V  3 


XE, 


ALTERNATING-CURRENT  MACHINERY 
or,  using  the  value  of  E2  from  equation  (41),  we  have 


333 


(42) 


It  is  to  be  noted  that  in  order  to  make  a  direct-current  machine 
into  a  three-,  four-,  or  six-ring  converter,  the  number  of  armature 
conductors  must  be  divisible  by  the  number  of  rings.  Therefore,' 
the  armature  shown  in  Fig.  301  is  not  suitable  for  a  four-ring  con- 
verter, although  it  is  suitable  for  a  three-ring  converter. 

The  foregoing  discussion  shows  that  if  the  effective  value  of 
E2  is  represented  by  the  diameter  of  a  circle,  then  the  effective  value 
of  Es  is  represented  by  a  120-degree  chord,  the  effective  value  of  E* 


Fig.  304.     E.M.F.  Relations  in  a    Four- 
Ring  Converter 


Fig.  305.      E.M.F.   Relations   in   a  Six- 
Ring  Converter 


is  represented  by  a  90-degree  chord,  and  the  effective  value  of  E& 
is  represented  by  a  60-degree  chord  of  the  same  circle.    This  is  shown 
in  Figs.  303,  304,  and  305. 
From  Fig.  304,  we  have 


or,  substituting  the  value  of  E2  from  equation  (41),  we  have 

1  E          E 


(43) 


334  ALTERNATING-CURRENT  MACHINERY 

From  Fig.  305  we  have 


or,  substituting  the  value  of  E2  from  equation  (41),  we  have 

(44) 


2 

Summary  of  E.  M.  F.  Relations  for  the  Rotary  Converter.  Let  E 
be  the  electromotive  force  between  the  direct-current  brushes  of  a 
rotary  converter;  then, 

E2  =  0.707  E  ' 

E3=  0.612  E 

E,  =  0.500  E 

E6=  0.354  E 


45) 


in  which  E^  Es,  E^,  and  E&  are  the  effective  values  of  the  alternating 
electromotive  force  between  adjacent  collecting  rings  on  a  two- 
ring,  three-ring,  four-  ring,  and  six-ring  converter,  respectively,  and  E 
is  the  steady  value  of  the  electromotive  force  between  the  direct- 
current  brushes  in  each  case. 

Examples.     If  a  rotary  converter  is  to  deliver  direct  current  at  500  volts  : 

(a)  It  must  be  supplied  with  single-phase  alternating  current  at  353.5 
volts  effective  if  it  is  a  two-ring  converter. 

(b)  It  must  be  supplied  with  three-phase  currents  at  306  volts  effective 
between  each  pair  of  the  three  supply  mains,  if  the  converter  is  a  three-ring 
converter. 

(c)  It  must  be  supplied  with  two-phase  currents  over  four-wire  supply 
mains  with  250  volts  effective  between  mains  connected  to  adjacent  collector 
rings,  or  353.5  volts  effective  between  mains  connected  to  opposite  collector 
rings,  if  the  converter  is  a  four-ring  converter. 

(d)  It  must  be  supplied  with  six-phase  currents  over  six-wire  supply 
mains,  with  177  volts  effective  between  the  mains  connected  to  adjacent  col- 
lector rings;  or  with  306  volts  effective  between  the  mains  connected  to  rings  1 
and  3;  or  with  353.5  volts  effective  between  the  mains  connected  to  opposite 
collector  rings. 

Modification  of  Theoretical  Voltage  Ratios  in  Actual  Machines. 
The  theoretical  ratios  of  alternating  to  direct-current  voltage  are 
not  always  found  to  hold  good  in  practice.  This  is  owing  to  a  variety 
of  causes,  chief  among  which  are:  the  deviation  of  the  generator 
voltage  from  a  sine  wave;  the  voltage  drop  in  the  armature  wind- 


ALTERNATING-CURRENT  MACHINERY 


335 


TABLE  X 

Voltage  Ratios  of  Rotary  Converters 


PERCENTAGE  POLE  ARC 

50  Per  Cent 

67  Per  Cent 

74  Per  Cent 

80  Per  Cent 

550-volt 

67 

63 

62 

61.5 

Three-phase 

250-volt 

67.5 

63.5 

62.5 

62 

125-volt 

68 

63.8 

63 

62^  _ 

550-volt 

78 

73.5 

72.5 

72 

Two-phase 

250-volt 

79 

74 

73 

72.5 

125-volt 

79.5 

74.5 

73.5 

73 

ings;  the  position  of  the  direct-current  brushes  on  the  commutator; 
and  the  degree  of  field  excitation. 

As  the  direct-current  voltage  at  the  commutator  brushes, 
neglecting  the  resistance  drop  IR  in  the  converter,  is  equal  to  the 
maximum  instantaneous  voltage  between  opposite  collector  rings, 
a  flat-top  wave  gives  a  higher  ratio,  i.  e.,  lower  direct-current  volt- 
age, and  a  peaked  wave  a  lower  ratio,  i.  e.,  higher  direct-current 
voltage,  for  the  same  impressed  alternating-current  voltage.  More- 
over, the  shape  of  the  electromotive  force  wave  impressed  by  the 
generator  upon  the  converter,  is  modified  by  the  form  of  the  counter- 
electromotive  force  wave  of  the  converter.  Hence,  a  short  pole  arc 
of  the  converter,  producing  a  peaked  wave  of  counter-electromotive 
force,  tends  to  lower  the  direct-current  voltage,  and  a  long  pole  arc 
tends  to  raise  the  direct-current  voltage,  for  the  same  impressed 
alternating  voltage. 

A  displacement  of  the  brushes  from  the  neutral  point  decreases 
the  direct-current  voltage  for  a  given  alternating-current  voltage, 
the  variation  in  extreme  cases  amounting  to  several  per  cent. 

Over-excitation  of  the  field  magnet  may  increase  the  direct- 
current  voltage  one  or  two  per  cent;  while  with  under-excitation,  i.  e., 
with  lagging  current  in  the  armature,  the  direct-current  voltage  may  be 
decreased  one  or  two  per  cent  for  a  given  alternating-current  voltage. 

Under  average  conditions  of  full-load  operation,  the  standard 
types  of  converters  have  ratios  alternating-current  voltage  -^direct- 
current  voltage,  approximately  as  given  in  Table  X. 

In  the  normal  operation  of  the  rotary  converter,  namely,  when 
furnishing  direct  current,  the  drop  in  the  armature  reduces  the 
direct-current  voltage.  When  run  as  an  inverted  converter,  i.  e.t 


336  ALTERNATING-CURRENT  MACHINERY 

when  delivering  alternating  current,  direct  current  being  fed  to  the 
brushes,  the  drop  is  on  the  alternating-current  side;  consequently 
the  voltage  ratio  of  a  converter  is  lower  when  it  is  run  inverted. 

For  preliminary  calculations  where  the  data  of  operation  is 
not  known,  the  following  voltage  ratios  may  be  used  with  most 
standard  converters: 

No  load       Full  load 

For  single-phase  71.5  73 

For  two-phase  71.5  73 

For  three-phase  61  62.5 

For  six-phase  71.5  73 

(measured  on  diameter) 

For  six-phase  61  62.5 

(measured  on  alternate  rings) 

In  operating  rotary  converters,  it  is  customary  to  make  allowance 
for  the  departure  of  the  actual  voltage  ratio  from  the  normal  ratio. 
The  amount  of  the  allowance  to  be  made  cannot  always  be  pre- 
determined; but  any  ordinary  departure  from  the  theoretical  volt- 
age may  be  easily  compensated  for  by  using  transformers  provided 
with  taps  on  the  secondary  windings  which  will  permit  a  voltage 
change  of  about  5  per  cent. 

Current  Relations  for  Rotary  Converter.  The  rotary  converter, 
as  ordinarily  used  to  convert  alternating  current  to  direct  current, 
behaves  as  a  synchronous  motor  so  far  as  its  intake  of  alternating 
current  or  currents  is  concerned.  In  the  case  of  the  synchronous 
motor  with  a  given  belt  load,  the  intake  of  alternating  current  varies 
with  the  degree  of  field  excitation,  the  intake  of  current  being  a 
minimum  for  a  certain  field  excitation,  and  the  power  factor  nearly 
unity  (see  page  226).  In  the  case  of  the  rotary  converter  also,  the 
intake  of  alternating  current  or  currents  is  a  minimum,  and  its  power 
factor  is  unity  for  a  certain  field  excitation,  the  direct-current  output 
of  power  being  given.  Under  these  conditions  there  is  a  definite/ 
relation  between  the  direct-current  7  delivered  by  the  converter, 
and  the  effective  value  of  the  alternating  current  flowing  in  at  each 
collecting  ring.  In  fact 

72  =  1.4147 

73  =  0.9437    , 

74  =  0.7077    > 
76  =  0.471  7 


ALTERNATING-CURRENT  MACHINERY  337 

in  which  72,  73,  I*>  and  76  are  the  effective  values  of  the  alternating 
current  entering  at  each  collector  ring  of  a  two-ring,  three-ring, 
four-ring,  or  six-ring  converter,  respectively;  and  7  is  the  direct 
current  delivered  by  the  converter.  These  equations  (46)  are  based 
on  the  assumption  that  the  converter  has  unity  power  factor  as  above 
pointed  out,  and  that  the  intake  of  power  is  equal  to  the  output  of 
power,  i.  e.,  the  losses  of  power  in  the  machine  are  ignored. 

Derivation  of  the  Equations  for  72  and  73.  The  method  of  deriv- 
ing equations  (46)  will  be  sufficiently  indicated  by  deriving  the  first 
two,  namely,  the  equations  for  72  and  73. 

The  direct-current  output  of  power  from  the  converter  is  El,  E 
being  the  electromotive  force  between  the  direct-current  brushes, 
and  7  being  the  direct  current  delivered.  The  intake  of  power  is 
EzIzX power  factor;  but  since  the  power  factor  is  supposed  to  be 
unity,  the  intake  of  power  is  simply  E272.  Therefore,  ignoring  losses 
of  power,  we  have 

E272  =  El 

But  E2=  0.707  E,  by  the  first  of  equations  (45),  so  that  E272  = 
0.707E  X  72  =  EL  Hence 

0.707  72=  7 
or,  in  final  form,  the  equation  for  72  is 

72=  1.4147 

The  direct-current  output  of  power  is  El,  and  the  intake  of 
power  is  I7  3  E3IS,  the  power  factor  being  unity;  that  is,  the  power 
delivered  by  three-phase  supply  mains  is  equal  to.V7  3  times  the 
voltage  between  mains  times  the  current  in  each  main,  as  explained 
on  page  128.  Therefore,  ignoring  power  losses  in  the  machine,  we  have 

1/TE373  =  E7 

But  E3  =  0.612  E,  according  to  the  second  of  equations  (45),  so  that 
^T  E3I3  =  T/T  x  0.612  X  El 3  =  EL  Hence 

y  Yx  0.612  73=7 
or,  in  final  form,  the  equation  for  73  is 

73  =  0.943  7 

Example.  A  rotary  converter  delivers  500  amperes  of  direct  current, 
the  field  excitation  being  adjusted  so  that  the  intake  of  alternating  current 
may  be  a  minimum. 


338  ALTERNATING-CURRENT  MACHINERY 

(a)  If  this  converter  is  a  two-ring  converter,  it  must  be  supplied  with 
707  amperes  of  alternating  current  from  single-phase  mains. 

(b)  If  this  converter  is  a  three-ring  converter,  it  must  be  supplied  with 
three-phase  currents  from  three-wire  mains,  with  471.5  amperes  effective  in 
each  main. 

(c)  If  this  converter  is  a  four-ring  converter,  it  must  be  supplied  with 
two-phase  currents  from  four-wire  supply  mains,  with  353.5  amperes  effective 
in  each  main. 

(d)  If  this  converter  is  a  six-ring  converter,  it  must  be  supplied  with 
six-phase  currents  from  six-wire  mains,  with  235.5  amperes  effective  in  each 
main. 

ROTARY  CONVERTERS  IN  PRACTICE 

Uses  of  the  Rotary  Converter.    The  uses  of  the  rotary  con- 
verter are  as  follows: 

(a)  Direct-Current    Generator    or    Motor.    The    rotary    con- 
verter may  be  used  as  a  direct-current  generator  or  motor,  in  which 
cases  the  collector  rings  are  not  used,  but  the  -machine  must  be 
provided  with  a  pulley. 

(b)  Alternating-Current  Generator  or  Synchronous  Motor.   The 
machine  may  be  driven  by  belt  and  used  to  deliver  alternating 
currents  from  its  collector  rings  as  an  ordinary  alternator;  or  the 
machine  may  be  supplied  through  its  collector  rings  with  alternat- 
ing currents,  and  may  be  driven  as  an  ordinary  synchronous  motor 
delivering  mechanical  power  to  drive  machinery.    In  either  case 
the  direct  current  for  exciting  the  field  magnet  may  be  taken  from 
the  commutator  of  the  machine,  or  from  a  separate  direct-current 
dynamo  (exciter). 

(c)  Double-Current  Generator.    The  rotary  converter  may  be 
driven  by  belt,  and  used  to  deliver  both  direct  current  and  alter- 
nating current  from  commutator  and  collector  rings,  respectively. 
When  so  used,  the  machine  is  called  a  double-current  generator. 

(d)  Regular  Rotary  Converter.     The  machine  may  be  driven 
as  a  synchronous  motor  taking  alternating  current   from    supply 
mains,  and  delivering,  not  mechanical  power,  but  electrical  power 
in  the  form  of  direct  current  from  its  commutator.    When  so  used, 
the  machine  is  called  a  rotary  converter.    This  is  the  most  frequent 
use  of  the  machine.    The  rotary  converter  does  not  require  a  pulley. 
Under  these  conditions,  the  machine  exhibits  all  of  the  peculiarities 
of  the  synchronous  motor.    Thus,  with  under-excited  field  magnet, 
the  rotary  converter  takes  an  unduly  large  amount  of  alternating 


ALTERNATING-CURRENT  MACHINERY  339 

current  from  the  supply  mains  at  a  low  power  factor.  As  the  field 
excitation  is  increased,  the  intake  of  alternating  current  decreases 
(for  given  output  of  direct-current  power),  and  the  power  factor  in- 
creases. For  a  certain  degree  of  field  excitation  the  power  factor 
is  nearly  unity,  and  the  alternating  current  or  currents  delivered 
to  the  machine  are  in  phase  with  the  alternating  electromotive  forces 
between  the  supply  mains.  When  the  field  magnet  is  over-excited, 
the  alternating  currents  supplied  to  the  converter  are  ahead  of  the 
alternating  electromotive  forces  in  phase,  and  the  power  factor  is 
less  than  unity. 

(e)  Inverted  Rotary  Converter.  The  machine  may  be  driven 
as  a  direct-current  motor  taking  current  from  direct-current  supply 
mains  through  its  commutator,  and  delivering,  not  mechanical 
power,  but  electrical  power  in  the  form  of  alternating  currents 
from  its  collector  rings.  When  so  used,  the  machine  is  called  an 
inverted  rotary  converter;  it  does  not  require  a  pulley. 

Starting  Rotary  Converters.  There  are  three  methods  in  common 
use  for  starting  rotary  converters,  namely,  (a)  from  the  alternating- 
current  side,  as  an  induction  motor;  (b)  from  the  direct-current 
side,  as  a  shunt  motor;  or  (c)  by  means  of  a  special  direct-connected 
starting  motor,  usually  of  the  squirrel-cage  induction  type. 

(a)  As  an  Induction  Motor.  The  first  method  is  one  of  the 
most  common  and  has  been  fully  discussed  on  page  209.  It  consists 
in  supplying  polyphase  currents  at  a  suitable  voltage  to  the  collector 
rings.  The  resulting  torque  developed  by  the  armature  will  bring 
the  converter  up  to  synchronous  speed  in  from  one  to  two  minutes. 
The  proper  value  of  impressed  voltage  for  starting  is  usually  obtained 
from  taps  on  the  main  transformers.  This  method  of  starting  has 
the  advantage  of  making  synchronizing  unnecessary,  but  on  the 
other  hand  it  requires  a  large  starting  current  which,  if  the  converter 
is  relatively  large  as  compared  with  the  alternator  supplying  it, 
causes  an  excessive  drop  of  voltage  throughout  the  circuit. 

A  difficulty  sometimes  met  in  this  method  of  starting  is,  that 
a  particular  direct-current  brush  or  set  of  brushes  may  be  positive 
or  negative  according  to  the  direction  of  the  last  pulse  of  alternating 
current  just  before  the  machine  jumps  into  synchronism.  There- 
fore, when  several  rotary  converters  are  to  supply  direct  current  to 
common  bus  bars,  special  care  must  be  taken  to  see  that  the  polarity 


340  ALTERNATING-CURRENT  MACHINERY 

of  a  given  machine  is  correct  before  it  is  connected  to  the  bus  bars. 
If  the  machine  happens  to  come  up  to  the  speed  with  the  wrong 
polarity,  the  field  "break-up"  switch,  described  on  page  220,  must  be 
thrown  from  one  position  to  the  other,  thus  reversing  the  field.  Self- 
starting  converters  are  also  generally  equipped  with  a  switch  for 
disconnecting  the  shunt  across  the  series  coils  from  the  series  field 
winding,  during  starting,  so  as  to  prevent  the  circulation  of  a  large 
induced  alternating  current  in  both  the  shunt  and  the  series  field 
winding.  The  induced  current  would  not  only  cause  excessive  heat- 
ing, but  would  also  hinder  starting  by  its  braking  effect. 

(b)  As  a  Shunt  Motor.  When  a  direct-current  supply  is 
available,  the  converter  may  be  started  as  a  direct-current  shunt 
motor,  by  supplying  direct  current  to  the  commutator  side  of  the 
machine,  the  alternating-current  main  switch  being  open.  \  This 
method  is  convenient  and  is  used  in  many  installations.  In  some 
cases  the  direct  current  is  obtained  from  another  converter  already 
in  operation,  or  from  a  small  storage  battery  which  may  be  at  hand. 
Sometimes  a  small  motor-generator  set,  consisting  of  an  induction 
motor  coupled  to  a  direct-current  generator,  is  installed  to  supply 
current  for  the  starting  of  one  or  more  converters  in  a  station. 

In  the  direct-current  method  of  starting,  the  fields  should 
be  fully  excited  by  closing  the  field  switch  first,  and  there  should 
be  a  resistance  in  series  with  the  armature  when  the  motor  switch 
is  closed.  Failure  to  excite  the  field  may  cause  the  converter  to 
increase  its  speed  to  a  dangerous  extent,  just  as  in  the  case  of  a  direct- 
current  shunt  motor  with  excessively  weak  field,  for  in  starting 
the  converter  from  the  direct-current  side,  it  is  not  running  as  a 
synchronous  motor  but  as  a  simple  shunt  direct-current  motor. 
If  the  converter  is  compound  wound,  the  series  field  circuit  must  be 
opened,  otherwise  the  current  flowing  through  it  will  magnetize  the 
field  poles  in  opposition  to  the  shunt  field  windings  (differential  com- 
pounding) and  may  even  prevent  the  machine  from  starting. 

The  operation  of  starting  is  as  follows: 

(1)  See  that  the  alternating-current  main  switch  is  open.  • 

(2)  Close  the  field  switch. 

(3)  Leave  the  starting  resistance  in  circuit  with  the  armature,  and 
then  close  the  direct-current  main  switch. 

(4)  When  normal  speed   is   reached,    cut   out   the  starting  rheostat, 


ALTERNATING-CURRENT  MACHINERY  341 

and  vary  the  field  strength  until  the  synchronizing  device  shows  that  the 
converter  is  in  synchronism  with  the  generator. 

(5)  Close  the  main  alternating-current  switch  when  the  synchronizing 
device  shows  that  the  rotary  converter  is  in  step  with  the  alternating-current 
supply. 

If  the  converter  is  furnished  with  a  shunt  winding  only,  adjust  the  field 
to  give  minimum  alternating-current  input. 

If  the  converter  has  a  series  winding  also,  the  shunt  field  should  be  ad- 
justed to  give  at  no  load  the  direct-current  no-load  voltage  at  which  it  is  rated. 

When  a  rotary  converter  is  started  as  a  direct-current  motor, 


Fig.  306.     Westinghouse  Rotary  Converter  Equipped  with  Separate  Starting 
Motor  on  Same  Shaft 

it  is  easy  to  bring  the  machine  into  operation  with  a  particular 
direct-current  brush  or  set  of  brushes  positive. 

(c)  By  Separate  Motor.  The  third  method  of  starting  rotary 
converters,  by  means  of  a  separate-starting  motor,  is  often  used. 
In  such  cases  the  starting  motor  is  usually  supported  on  the  con- 
verter either  on  one  end  of  the  base  plate  or  by  the  pillow  block. 
The  rotor  is  mounted  on  the  armature  shaft  of  the  converter  just 
outside  of  the  bearing,  as  shown  in  Fig.  306.  The  starting  motor 
is  usually  of  the  induction  type  with  squirrel-cage  rotor,  and  is 


342  ALTERNATING-CURRENT  MACHINERY 

furnished  with  a  number  of  poles  which  is  two  less  than  the  number 
of  poles  on  the  converter.  This  enables  the  motor  to  bring  the  con- 
verter up  to  and  above  the  synchronous  speed,  and  speed  regulating 
devices  are  arranged  for  synchronizing  the  converter  with  the  alter- 
nating-current supply  mains. 

This  method  of  starting  has  the  great  advantage  of  requiring 
a  relatively  small  starting  current,  and  is  used  especially  when 
because  of  limited  capacity  of  generators  or  transmission  system 
it  is  essential  to  keep  the  starting  current  as  low  as  possible. 

The  disadvantages  of  this  method  are :  that  it  requires  time  and 
skilled  attendants  to  synchronize  properly,  and  that  the  auxiliary 
motor  adds  to  the  cost  and  requires  additional  space. 

A  modification  of  the  last r  method  is  sometimes  employed  in 
which  the  converter  is  started  from  rest  by  means  of  a  separate 
starting  motor  and  then  thrown  directly  on  to  the  alternating- 
current  busses  in  series  with  a  suitable  reactance  which  limits  the 
instantaneous  rush  of  current  to  a  safe  value.  This  method  com- 
bines the  advantages  of  separate  motor  starting  and  self-starting 
from  the  alternating-current  side,  in  that  it  obviates  the  necessity 
of  synchronizing  and  insures  the  rotary  coming  in  with  the  right 
polarity;  on  the  other  hand,  it  requires  somewhat  more  time  than 
the  self -starting  method  and  a  heavier  line  current  than  with  the 
induction  starting  motor  alone.  An  example  of  this  method  is 
shown  in  Fig.  306,  a  300-kilowatt  three-phase  rotary  converter 
manufactured  by  the  Westinghouse  Company.  It  has  10  poles 
and  when  supplied  with  alternating  three-phase,  60-cycle  currents, 
at  367  volts  per  phase,  delivers  a  direct  current  of  500  amperes  at 
600  volts  from  the  commutator.  This  converter  is  built  for  electric 
railway  substation  service,  and  is  provided  with  an  induction  motor 
for  separate  starting.  At  the  right-hand  end  of  the  shaft  may  be 
seen  the  mechanical  oscillator  described  below. 

Oscillators  for  Rotary  Converters.  The  armature  of  a  belted 
dynamo  or  motor  is  always  caused  by  the  belt  to  shift  slowly  to  and 
fro  endwise  in  its  bearings,  thus  entirely  obviating  the  uneven  wear- 
ing away  of  the  commutator  in  grooves  where  the  brushes  rub. 
The  rotary  converter,  however,  not  being  mechanically  driven 
tends  to  run  without  end-play,  and  some  special  end-play  device, 
or  oscillator,  is  necessary.  An  effective  device  much  used  is  an  electro- 


ALTERNATING-CURRENT  MACHINERY  343 

magnet  mounted  opposite  to  the  end  of  the  rotary-converter  shaft. 
This  electromagnet  is  excited  about  ten  times  per  minute,  and  at 
each  time  gives  an  endwise  pull  on  the  shaft,  causing  the  desired 
endwise  movement  of  the  latter.  A  mechanical  end-play  device 
regularly  used  by  the  Westinghouse  Company  consists  of  a  steel 
plate  with  a  grooved  ball  race  and  ball  backed  by  a  spring,  and 
mounted  at  one  end  of  the  shaft.  As  the  grooved  plate  is  not_ 
quite  parallel  to  the  end  of  the  shaft,  when  the  converter  is  prop- 


Fig.  307.     General  Electric  300-Kilowatt  Three-phase  Rotary  Converter 

erly  installed  with  the  armature  slightly  inclined  towards  the  oscil- 
lator, the  hardened  steel  ball  is  caught  at  the  lowest  point  be- 
tween the  race  and  the  end  of  the  shaft.  The  ball  is  carried  upward 
as  the  armature  revolves  and  the  spring  is  compressed.  The  reaction 
of  the  spring  now  forces  the  shaft  away  and  the  ball  falls  back  to  its 
normal  position. 

Characteristic  Types  of  Rotary  Converters.  Fig.  307  shows  a 
300-kilowatt,  three-phase  (three-ring)  rotary  converter  built  by  the 
General  Electric  Company.  It  has  a  six-pole  field  magnet  and 
six  sets  of  direct-current  brushes,  each  set  having  eight  single  brushes. 


344 


ALTERNATING-CURRENT  MACHINERY 


Its  rated  speed  is  500  revolutions  per  minute,  which,  with  a  six-pole 
field,  gives  a  frequency  of  25  cycles  per  second.  The  three  collector 
rings  are  mounted  on  the  armature  shaft  on  the  end  opposite  to  the 
commutator.  It  can  be  seen  from  the  figure  that  the  commutator 
is  larger  in  comparison  with  the  size  of  the  machine  than  is  usual 
in  an  ordinary  direct-current  generator.  Each  collector  ring  has 
three  brushes  bearing  upon  it  in  order  to  permit  of  the  delivery  to 
the  machine  "of  the  very  large  alternating  currents  from  the  supply 


Fig.  308.     General  Electric  Three-Phase  Rotary  Converter  for  Electric  Light  Service 

mains.  The  use  of  three  brushes  on  each  collector  ring  is  preferable 
to  the  use  of  one  broad  brush,  inasmuch  as  it  is  desirable  to  make 
the  rings  narrow  to  save  space.  This  machine  is  rated  at  550  volts  be- 
tween its  direct-current  brushes,  so  that  the  full-load  direct-current 
output  is  300,000  watts  divided  by  550  volts,  or  546  amperes.  There- 
fore, on  the  assumption  of  unity  power  factor  and  100  per  cent 
efficiency,  as  explained  on  page  336,  the  alternating  current  entering 
at  each  collector  ring  is  546  amperes  X  0.943  =  515  amperes  effective; 
and  the  effective  voltage  between  collector  rings  is  550  volts  X  0.612* 
=  336  volts.  The  alternating-current  power  supplied  to  the  machine 
is 

VT  X  336  volts  X  515  amperes  =  300,000  watts 

*See  Equation  (42),  page  333. 


ALTERNATING-CURRENT  MACHINERY  345 

Fig.  308  shows  a  three-phase  rotary  converter  manufactured 
by  the  General  Electric  Company  for  electric  lighting  service  at 
250  volts.  The  characteristic  features  which  distinguish  the  rotary 
converter  from  the  direct-current  generator  are  here  especially 
prominent,  namely,  the  large  commutator  and  great  brush  contact 
area,  the  comparatively  large  collector  rings,  and  the  relatively 
small  magnetic  system,  including  yoke  and  field-magnet  cores. 

Hunting  of  Rotary  Converter.  A  rotary  converter  (regular), 
being  a  synchronous  motor  in  relation  to  its  alternating-current 
supply,  has  a  tendency  to  hunt,  as  explained  on  page  223. 

A  rotary  converter,  however,  having  no  pulley  and  not  being 
mechanically  connected  to  machinery,  is  much  more  sensitive  in 
responding  to  the  pulsations  of  an  engine  or  to  other  causes  of  hunt- 
ing than  is  a  synchronous  motor  delivering  mechanical  power. 

The  hunting  oscillations  of  a  synchronous  motor  are  always  due 
to  external  disturbances,  that  is,  to  disturbances  originating  outside  of 
the  alternating-current  generator,  the  line,  and  the  synchronous  motor. 

A  sudden  change  of  load  on  the  synchronous  motor,  for  ex- 
ample, is  followed  by  a  series  of  oscillations.  Whether  or  not  the 
oscillations  due  to  a  certain  change  of  load  give  rise  to  trouble, 
depends  largely  upon  the  resistance  and  the  reactance  of  the  trans- 
mission line,  and  upon  the  frequency.  The  greater  the  resistance 
and  the  reactance  of  the  transmission  line,  the  greater  the  trouble 
from  hunting;  and  at  high  frequencies  the  trouble  from  hunting 
is  much  greater  than  at  low  frequencies.  Thus  a  25-cycle  rotary 
converter  gives  no  serious  trouble  from  hunting  if  the  line  resist- 
ance and  reactance  are  not  excessively  high,  whereas  a  60-cycle 
rotary  converter  is  more  likely  to  give  trouble  unless  special  pro- 
vision is  made  to  diminish  hunting,  as  explained  below. 

A  periodic  variation  in  the  speed  of  the  engine  driving  the 
alternator  from  which  a  rotary  is  supplied  writh  alternating  current, 
produces  very  troublesome  hunting  when  this  variation  of  speed  is 
in  rhythm  with  the  hunting  oscillations.  This  class  of  hunting  is 
obviated  by  increasing  the  fly-wheel  capacity  of  the  engine,  or  by 
changing  the  resistance  or  the  reactance  of  the  transmission  lines.  The 
latter  method  changes  the  rhythm  of  the  hunting  oscillations  and 
thereby  does  away  with  the  coincidence  of  rhythm,  which  is  the  chief 
cause  of  excessive  hunting  oscillations  due  to  engine  pulsations. 


346 


ALTERNATING-CURRENT  MACHINERY 


The  hunting  oscillations,  once  started,  usually  reach  their 
maximum  under  given  conditions  in  a  few  minutes  of  time,  so  that 
serious  trouble  due  to  hunting,  such  as  the  dropping  out  of  step  of 
the  rotary,  or  excessive  sparking  at  the  commutator,  usually  occurs 
soon  after  the  hunting  begins. 

Hunting  which  is  approximately  the  same  at  all  loads,  from  no- 
load  to  over-load,  is  more  troublesome  when  the  field  of  the  rotary 
converter  is  over-excited  so  as  to  take  leading  currents,  than  when 
the  excitation  is  such  as  to  give  either  unity  power  factor  or  lagging 
currents. 

Hunting  is  also  greater  when  several  rotaries  are  supplied  from 
an  alternating-current  generator,  than  when  a  single  rotary  con- 
verter is  supplied,  unless  there  are  short  lengths  of  alternating- 
current  mains  between  them,  or  unless  the  converters  supply  direct 
current  in  parallel  to  the  same  direct-current  mains. 

Dampers.  Hunting,  whether  due  to  engine  pulsations  or  to 
momentary  outside  disturbance,  such  as  sudden  change  of  load  or 
momentary  short-circuit,  is  greatly  reduced  by  the  use  of  massive 

copper  bridges  or  frames 


Fig.  309. 


Diagram  Showing  Copper  Dampers 
in  Place  in  Field  Poles 


these    holes     or     channels   heavy 


extending  partly  over  the 
pole  faces,  as  shown  in  Fig. 

'  199.  A  more  effective  ar- 
rangement is  shown  in  Figs. 
J  71  and  309.  A  number  of 
holes  or  channels  are  pro- 
vided in  each  pole  tip.  In 

copper    conductors   A    and  B 


are  placed,  and  these  conductors  are  short-circuited  by  being 
connected  at  the  ends  by  the  bars  C  C.  These  copper  frames  or 
"dampers"  diminish  hunting  oscillations  because,  when  a  machine 
is  hunting,  the  magnetic  flux  from  pole-face  to  armature  core 
is  shifted  forwards  and  backwards  over  the  pole-face,  and  this  shift- 
ing flux  induces  electromotive  forces  in  the  copper  frames,  which 
produce  currents  tending  to  oppose  the  shifting  of  the  flux,  and 
thereby  oppose  the  hunting  oscillations. 

A  rotary  converter  having  solid  cast-steel  pole  pieces  has  little 
or  no  tendency  to  hunt.  The  action  of  the  solid  poles  is  the  same 
as  the  action  of  the  massive  copper  conductors  shown  in  Fig.  309. 


ALTERNATING-CURRENT  MACHINERY  347 

The  use  of  solid  poles,  however,  leads  to  excessive  eddy-current 
losses;  hence  solid  poles  are  not  considered  desirable. 

Inverted  Rotaries.  When  a  rotary  converter  is  used  to  convert 
a  direct  current  into  an  alternating  current,  taking  direct  current 
in  at  the  commutator  and  delivering  alternating  current  at  the 
collector  rings,  it  is  called  an  inverted  rotary  converter.  While  the 
rotary  converter  is  generally  used  to  convert  alternating  current 
into  direct  current,  it  sometimes  happens  that  inverted  converters 
are  desirable.  For  example,  in  a  low-tension,  direct-current  system, 
a  district  remote  from  the  central  station  may  be  supplied  with 
current  by  converting  direct  current  to  alternating  current  at  the 
station  (by  means  of  inverted  rotaries);  then,  by  step-up  trans- 
formers, raising  the  voltage  to  a  high  value,  and  transmitting  it  as 
high-tension  alternating  current;  and  finally,  at  the  distant  point, 
reconverting  it  (using  step-down  transformers)  to  direct  current. 
Again,  in  a  station  containing  direct-current  generators  for  short- 
distance  supply,  and  alternators  for  long-distance  supply,  the  con- 
verter may  be  used  as  the  connecting  link  to  shift  the  load  from 
the  direct  to  the  alternating  generators,  or  conversely.  The  ma- 
chine which  is  used  for  shifting  the  load  in  this  way  is  caused  to 
operate  as  a  regular  rotary  or  as  an  inverted  rotary  according  to 
the  demand  for  direct  current  or  alternating  current. 

The  behavior  of  an  inverted  rotary  converter  is  different  in 
many  respects  from  the  performance  of  the  same  machine  when 
used  as  a  regular  rotary  converter.  When  converting  from  alter- 

(nating  current  to  direct  current,  the  speed  of  the  converter  is  rigidly 
fixed  by  the  frequency  of  the  alternating  current  supplied  to  it,  and 
cannot  be  varied  by  alternating  its  field  excitation.  Varying  the 
field  excitation  would  merely  change  the  phase  difference  between  the 
alternating  electromotive  force  and  current  supplied  to  the  machine, 
and  hence  the  power  factor,  as  in  the  case  of  the  synchronous  motor. 
When  converting  from  direct  current  to  alternating  current,  however, 
the  speed  of  the  converter,  as  in  a  direct-current  motor,  will  be  pro- 
portional to  the  applied  direct-current  voltage,  and  will  also  depend 
upon  the  field  excitation.  The  effect  of  weakening  the  field  is  to  in- 
crease the  speed,  and  the  effect  of  strengthening  the  field  is  to  de- 
crease the  speed.  It  is  evident,  therefore,  that  there  should  be 
little  or  no  series  field  winding  provided  on  an  inverted  rotary  con- 


348  ALTERNATING-CURRENT  MACHINERY 

verier,  as  it  will  change  in  speed  under  load  and  deliver  alternating 
currents  at  a  variable  frequency.  If  the  field  becomes  greatly 
weakened,  an  inverted  rotary  converter  may  reach  a  dangerously 
high  speed  before  the  attendant  has  time  to  prevent  it,  and  the 
armature  of  the  machine  may  be  torn  to  pieces  by  the  excessively 
large  centrifugal  forces. 

Changing  the  field  excitation  of  an  inverted  rotary  converter 
will  not  change  the  voltage  of  the  alternating  current,  because  the 
ratio  of  transformation  in  a  given  converter  is  fixed;  changing  the 
field  strength  merely  causes  a  change  in  the  speed  of  the  rotary. 

*  The  voltage  of  the  alternating  current  may  be  changed  by 
changing  the  voltage  of  the  applied  direct  current,  or  it  may  be 
varied  by  using  alternating-current  .potential  regulators,  page  448. 

Speed-Limiting  Devices.  An  inverted  rotary  being  an  alternating- 
current  generator,  its  field  strength  depends  upon  the  intensity  and 
the  phase  relation  of  the  alternating  current  supplied  by  its  armature; 
thus  a  lagging  current  in  the  armature  reduces  the  field  strength, 
and  hence  increases  the  speed  and  the  frequency;  whereas,  a  leading 
current  increases  the  field  strength,  and  thus  decreases  the  speed 
and  the  frequency.  Again,  if  the  alternating-current  side  of  an 
inverted  rotary  converter  delivers  large  lagging  currents  to  induc- 
tive receiving  circuits,  such  as  induction  motors,  the  demagnetizing 
action  of  the  lagging  armature  currents  on  the  field  may  result  in 
a  dangerously  high  speed.  In  operating  inverted  rotary  converters, 
therefore,  especially  when  they  are  liable  to  be  overloaded  on  the 
alternating-current  side,  as  in  the  starting  of  synchronous  or  induc- 
tion motors  supplied  from  the  inverted  rotary,  great  care  should  be 
taken  to  see  that  the  field  excitation  is  always  great  enough  to  pre- 
vent excessive  speeds.  When  used  for  the  above  purpose,  special 
speed-limiting  devices  should  be  used. 

A  method  used  by  the  Westinghouse  Company  to  prevent 
this  tendency  of  the  inverted  rotary  converter  to  race,  is  as  follows: 

The  converter  is  separately  excited  by  a  small  direct-current  generator 
mechanically  connected  to,  and  driven  by,  it,  The  speed  of  the  exciter  will, 
therefore,  change  with  every  change  in  the  speed  of  the  inverted  rotary.  The 
magnetic  circuit  (magnet  cores,  yoke,  etc.,)  and  magnet  coils  of  the  exciter 
are  so  designed  that  its  armature  can  generate  normal  voltage  when  the  machine 
is  being  worked  at  a  point  considerably  below  the  "knee"  of  the  saturation 
curve.  Any  increase  in  the  speed  of  the  exciter  will,  therefore,  cause  a  great 


ALTERNATING-CURRENT  MACHINERY  349 

increase  in  its  voltage.  If  then  the  speed  of  the  inverted  rotary  converter 
increases,  the  voltage  of  the  exciter  immediately  increases  and  strengthens 
the  field  of  the  converter,  thus  checking  its  tendency  to  race. 

The  same  result  is  attained  by  the  General  Electric  Company, 
but  in  a  different  manner,  as  follows: 

A  kind  of  centrifugal  governor  is  attached  to  the  shaft  of  the  inverted 
rotary  and  revolves  with  it.  If  the  speed  of  the  rotary  converter  increase^ 
above  a  certain  value,  the  governor  acts,  thus  closing  an  electric  circuit  which 
automatically  throws  off  the  power  supplied  to  the  rotary. 

Control  of  Direct=Current  Voltage.  The  relations  between  the 
alternating-current  voltages  supplied  to  a  rotary  converter  and  the 
direct-current  voltage  of  the  machine,  as  explained  on  page  331,  and 
as  expressed  in  equations  (45),  apply  to  the  case  in  which  the  field 
excitation  of  the  rotary  converter  is  such  that  the  alternating  cur- 
rents delivered  to  the  machine  are  in  phase  with  the  applied  alter- 
nating-current voltages;  that  is,  these  relations  apply  to  the  case 
in  which  the  power  factor  of  the  machine  is  unity.  The  direct- 
current  voltage  of  a  rotary  converter  may  be  slightly  greater  or 
less  than  the  ideal  value  given  in  equations  (45),  but  the  fact 
remains  that  the  ratio  of  the  alternating-current  voltage  to  the  direct- 
current  voltage  of  a  given  converter  is  nearly  constant.  The  fixed 
ratio  of  these  voltages  is  a  serious  handicap  in  electric  railway  and 
similar  work  where  it  is  desirable  to  have  the  station  voltage  increase 
as  the  demand  for  current  increases,  in  order  that  the  voltage  at  a 
distant  point  of  the  line  shall  be  kept  up  in  spite  of  the  increase  in 
line  drop.  In  order,  therefore,  to  increase  the  voltage  of  the  direct- 
current  output  of  the  rotary  converter  it  is  necessary  to  increase 
also  the  alternating-current  voltage  supplied  to  its  collector  rings. 
This  increase  of  voltage  may  be  secured  by  the  following  methods: 

(a)  Variable  ratio  step-down  transformers 

(b)  Variable  ratio  low-voltage  autotransformers 

(c)  Voltage  regulators  (see  page  448) 

(d)  Synchronous  regulators  or  boosters 

(e)  Series  field  winding  properly  proportioned  in  connection  with  series 
inductive  reactance    (compounding) 

Methods  (a),  (b),  (c),  and  (d)  are  non-automatic,  and  are  used 
where  the  load  is  fairly  constant  over  considerable  periods.  Method 
(e)  is  entirely  automatic  within  a  range  of  10  to  15  per  cent  and  is 
frequently  used  where  the  load  is  rapidly  fluctuating,  as  in  electric 


350 


ALTERNATING-CURRENT  MACHINERY 


railway  service.     With  the  necessary  auxiliary  apparatus,  methods 
(c)  and  (d)  can  be  made  to  operate  automatically. 

In  using  method  (a)  the  transformers  for  rotary  converter  service 
.are  designed  for  the  normal  secondary  voltage  which  will  give  the 
required  direct-current  voltage.  They  are  provided  with  taps  on 
both  primary  and  secondary  windings  which  allow  compensation 
to  be  made  for  line  drop  or  for  any  small  variation  from  the  desired 
standard  voltage.  These  taps,  however,  cannot  be  used  for  controlling 
the  voltage  while  the  apparatus  is  in  service.  Method  (d)  requires 
a  synchronous  booster  which  is  merely  an  auxiliary  alternating- 
current  generator,  with  the  same  number  of  poles  as  the  converter, 
and  whose  armature  is  mounted  on  the  shaft  of  the  converter.  The 
armature  winding  of  this  generator  is  connected  in  series  between 
the  armature  and  the  collector  rings  of  the  converter.  The  field 


Fig.  310.      Rotor  of  Westinghouse  Six-Phase  Rotary  Converter  with  Synchronous  Regultaor 

magnets  of  the  booster  are  separately  excited  so  that  its  armature 
voltage  may  be  varied  from  zero  to  a  maximum.  Thus,  since  the 
armature  windings  of  both  the  converter  and  the  booster  are  in 
series,  the  alternating-current  voltage  supplied  to  the  collector 
rings  may  be  varied  at  will,  and  the  voltage  on  the  direct-current 
side  changed  accordingly. 

If  necessary  the  excitation  and  the  polarity  of  the  regulator 
may  be  reversed,  so  that  the  voltage  derived  from  the  regulator 
may  be  subtracted  from  the  normal  value  of  the  impressed  alternat- 
ing-current voltage  instead  of  added  to  it,  and  a  corresponding 
reduction  of  the  direct-current  voltage  obtained.  The  regulator 
may  be  arranged  for  either  automatic  or  manual  control. 

With  a  synchronous  regulator  having  15  per  cent  of  the  rotary 
converter  capacity,  the  direct-current  voltage  can  be  varied  30  per 


ALTERNATING-CURRENT  MACHINERY 


351 


cent  at  will;  or,  if  desired,  a  constant  voltage  can  be  maintained. 
Moreover,  the  regulator  field  can  be  so  controlled  that  when  operating 
in  parallel  with  other  rotary  converters,  or  storage  batteries,  the  con- 
verter will  take  its  proper  share  of  the  total  load. 

Fig.  310  shaws  the  rotor  of  a  770-kw.,  six-phase  rotary  converter 
provided  with  a  synchronous  regulator  or  booster,  made  by  the  West- 
inghouse  Electric  Company.  Passing  from  left  to  right  in  the  figurer 
there  are :  the  commutator  of  the  converter,  the  armature  of  the  con- 
verter, the  armature  of  the  synchronous  regulator,  the  six  collector 


Fig.  311.     Westinghouse  Three-Phase  Converter  with  Synchronous  Regulator 

rings,  and  on  the  extreme  right  the  squirrel-cage  rotor  of  the  auxiliary 
starting  motor. 

Fig.  311  is  a  view  of  a  Westinghouse  three-phase  1,000-kw. 
converter  with  a  synchronous  regulator.  The  regulator  frame,  with 
its  field  poles  and  windings,  is  supported  by  brackets  attached  to  the 
converter  frame.  The  converter  itself  is  the  same  as  other  standard 
converters,  the  main  field  windings  being  free  from  all  regulating 
devices. 

Method  (e)  for  voltage  control  depends  on  the  action  of  the 
series  field  winding  in  connection  with  series  reactance.  As  already 


352  ALTERNATING-CURRENT  MACHINERY 

shown  on  page  226,  the  field  excitation  of  a  synchronous  motor  may 
be  varied  through  quite  a  range  above  or  below  that  corresponding 
to  unity  power  factor,  the  machine  taking  leading  currents  when  its 
field  is  over-excited,  and  lagging  currents  when  its  field  is  under- 
excited.  This  is  especially  the  case  when  the  synchronous  motor  has 
considerable  armature  inductance,  and  when  the  transmission  line 
also  has  considerable  inductance.  This  remark  applies  to  the  rotary 
converter  also;  and,  when  the  transmission  line  has  considerable 
reactance,  the  alternating-current  voltages  between  the  collector 
rings  of  a  rotary  converter  and  the  direct-current  voltage  between 
its  direct-current  brushes  vary  with  the  field  excitation  of  the  con- 
verter. 

Where  there  is  both  inductance  and  resistance  drop  in  the 
feeders  and  a  considerable  variation  in  the  alternating  voltage 
supply,  the  converter,  if  provided  with  series  field  winding,  can 
be  made  to  regulate  automatically  for  constant  direct-current  volt- 
age within  reasonable  limits,  as  explained  below. 

As  the  ratio  of  the  alternating  current  to  the  direct-current 
voltage  of  a  converter  is  nearly  constant,  the  impressed  alternating 
voltage  must  be  varied  in  order  to  vary  the  direct-current  voltage. 
This  can  be  done  by  taking  advantage  of  the  fact  that  an  alternating 
current  passing  over  an  inductive  circuit  will  decrease  in  voltage 
if  lagging  in  phase  behind  its  electromotive  force,  and  will  increase 
in  voltage  if  leading.  Just  as  in  the  case  of  a  synchronous  motor,  a 
certain  field  excitation  in  any  converter  will  give  a  minimum  arma- 
ture current.  If  the  excitation  be  decreased,  the  armature  current 
will  be  increased  but  will  be  lagging.  By  providing,  therefore, 
sufficient  reactance  in  the  alternating-current  circuit  connecting  a 
converter  with  its  source  of  power,  the  alternating-current  voltage 
at  the  converter  terminals  may  be  varied  by  means  of  the  field  exci- 
tation of  the  converter,  and  without  altering  the  generator  voltage. 

When  it  is  desired  to  control  the  direct-current  voltage  of  a 
rotary  converter  independently  of  the  voltage  of  the  alternating- 
current  generator  that  supplies  the  alternating  currents,  the  trans- 
mission line  is  frequently  given  an  artificial  reactance  by  connecting 
inductance  (reactance)  coils  in  series  with  the  alternating-current 
supply  mains.  Thus,  the  alternating  currents  delivered  to  the  con- 
verter are  caused  to  flow  through  these  reactance  coils.  When, 


ALTERNATING-CURRENT  MACHINERY  353 

therefore,  a  rotary  converter  has  a  compound  field  winding  (series 
and  shunt),  as  described  below,  the  use  of  reactance  coils  in  the 
supply  mains  is  necessary  if  the  transmission  lines  do  not  of  them- 
selves have  sufficient  reactance. 

Field  Excitation.  Various  methods  are  employed  for  exciting 
the  field  magnet  of  rotary  converters,  as  follows: 

(a)  By    Armature    Reaction.      WJien    an    alternating-current 
generator  delivers  leading   current  to  a  receiving  circuit,  the  mag- 
netizing action  of  the  armature  currents  tends  to  strengthen  the  field 
magnet  poles,  as  explained  on  page  109.     If  an  alternating-current 
generator  were  always  used  to  deliver  leading  currents,  it  would  be 
possible  to  depend  upon  the   magnetizing  action  of  the  armature 
currents  entirely  for  exciting  the  field  magnet,  without  using  any 
direct  current  whatever  in  the  field  windings;  in  fact,  the  field  windings 
could  be  dispensed  with  altogether. 

When  a  synchronous  motor,  or  rotary  converter,  takes  lagging 
currents  from  alternating-current  supply  mains,  the  magnetizing 
action  of  these  currents  in  the  motor  armature  is  to  strengthen  the 
field  magnetism  of  the  motor,  or  rotary  converter.  This  action  alone 
may  be  utilized  for  exciting  the  field  magnet  of  a  synchronous  motor, 
or  rotary  converter,  and  rotary  converters  have  been  designed  and 
commercially  operated  with  this  mode  of  field  excitation. 

(b)  Self -Excitation  by  Direct-Current  Taken  from  the  Commu- 
tator of  the  Machine.    The  usual  method  of  exciting  the  field  of  a 
rotary  converter  is  by  means  of  direct  current  taken  from  the  com- 
mutator of  the  machine  itself.    There  are  three  schemes  for  carrying 
out  this  method  of  field  excitation,  exactly  as  in  the  case  of  ordinary 
direct-current  generators,  as  follows: 

(1)  Series  excitation.     This  scheme,  in  which  the  entire  direct-current 
output  flows  through  the  field  winding  (coarse  wire),  gives  a  field  excitation 
which  is  zero  when  the  direct-current  output  is  zero,  and  which  rises  to  full- 
rated  excitation  when  full-load  output  of  direct  current  is  reached.     This 
scheme  of  field  excitation  is  not  suitable  for  rotary  converters,  inasmuch  as  a 
rotary  converter  should  have  an  approximately  constant-field  excitation,  or  a 
field    excitation    which    changes    through    a    comparatively    narrow    range 
only. 

(2)  Shunt  excitation.     In  this  scheme  the  field  winding  is  made  of  com- 
paratively fine  wire.     Its  resistance,  therefore,  is  comparatively  high,  and  it 
is  connected  directly  between  the  direct-current  brushes  with  an  adjustable 
field  rheostat  in  its  circuit,  exactly .  as  in  the  ordinary  shunt-wound  direct- 


354  ALTERNATING-CURRENT  MACHINERY 

current  dynamo.  This  scheme  gives  an  approximately  constant  field  excitation, 
and  it  is  much  used  in  rotary  converters.  The  variation  of  field  excitation  for 
the  purpose  of  controlling  the  power  factor  of  the  converter  is  accomplished 
by  means  of  the  adjustable  field  rheostat. 

(3)  Compound  excitation.  The  combination  of  series  and  shunt  excita- 
tion is  frequently  used  in  rotary  converters  so  as  to  provide  for  slightly  increas- 
ing field  excitation  (by  means  of  the  series  winding)  with  increasing  direct- 
current  output.  This  scheme  of  field  excitation  is,  however,  more  limited 
when  applied  to  a  rotary  converter  than  when  applied  to  an  ordinary  direct- 
current  dynamo,  for  the  reason  that  too  great  an  increase  of  field  excitation 
in  a  rotary  converter  (as  in  the  case  of  a  synchronous  motor)  causes  the  con- 
verter to  fall  out  of  synchronism  and  stop,  or  "break  down,"  as  it  is 
termed. 

Compound-wound  rotary  converters  are  used  to  advantage  for 
supplying  current  which  is  constantly  fluctuating  (and  where  the 
generators  supplying  the  converters  do  not  greatly  exceed  the  latter 
in  kilowatt  capacity)  as  in  railway  service,  and  in  cases  where  it  is 
necessary  to  maintain  constant  or  increasing  voltage  with  increasing 
load.  More  or  less  prominence  can  be  given  to  shunt  or  series  wind- 
ings as  may  be  required. 

The  regulation  is  made  automatic  by  a  series  field  winding  on 
the  converter,  but  the  inductance  of  the  transmission  lines  and  gen- 
erator must  frequently  be  increased  by  introducing  reactive  coils. 

The  amount  of  raising  ("boosting")  or  lowering  of  the  volt- 
age is  proportional  to  the  reactance  in  circuit,  for  a  given  series  field. 
Considering,  however,  that  the  maximum  output  of  the  converter 
and  its  stability  are  affected  by  too  much  reactance,  the  introduc- 
tion of  reactance  should  not  be  carried  too  far. 

In  compound-wound  converters  the  shunt  excitation  is  gen- 
erally adjusted  to  give  a  lagging  current  of  from  20  per  cent  to  30 
per  cent  of  full-load  current  at  no  load  by  under-excitation,  and  the 
series  field  is  adjusted  to  give  a  slightly  leading  current  at  full  load. 
This  arrangement  lowers  the  impressed  voltage  at  the  converter  at 
no  load  and  raises  it  at  full  load  enough  (with  constant  voltage  at  the 
generator)  to  compensate  for  all  the  losses  of  voltage  in  the  system, 
thus  making  possible  the  delivery  of  a  constant  direct-current  voltage 
at  all  loads. 

It  has  been  found  in  practice  that  the  compound  winding  dis- 
tinctly diminishes  the  stability  of  running  when  the  tendency  to 
hunt  is  present  to  any  extent.  The  series  winding  should  be  cut  out 


ALTERNATING-CURRENT  MACHINERY  355 

when  starting  up  from  the  direct-current  side.  This  is  conveniently 
accomplished  by  a  double-throw  switch  which  in  one  position  con- 
nects the  junction  of  the  series  winding  and  the  negative  brushes 
to  the  starting  rheostat,  and  in  the  other  position  connects  this 
junction  with  the  equalizing  bar. 

(c)  Separate  Excitation.  The  use  of  a  small  auxiliary  direct- 
current  dynamo  to  supply  direct  current  for  exciting  the  field  of  .a 
rotary  converter  has  been  mentioned  on  page  348,  where  it  was 
pointed  out  that  this  method  of  field  excitation  is  especially  suited 
to  inverted  rotaries. 

Rotary  Converter  with  Edison  Three=Wire  System.  The 
Edison  three-wire  system  must  ordinarily  be  supplied  with  current 
from  two  direct-current  generators  connected  in  series  between  the 
outside  mains,  and  with  the  neutral  main  connected  to  the  junction 
of  the  two  machines. 

To  operate  a  three-wire  system  from  a  single  direct-current 
generator  would  give  rise  to  great  differences  of  voltage  on  the  two 
sides  of  the  system  when  the  number  of  lamps  on  one  side  differs 
greatly  from  the  number  of  lamps  on  the  other  side;  in  fact,  it  would 
not  be  allowable  to  turn  off  a  lamp  on  one  side  without  turning  off 
a  lamp  on  the  other  side  at  the  same  time.  To  avoid  this  difficulty 
some  arrangement  which  is  equivalent  to  the  use  of  two  generators 
is  necessary. 

A  rotary  converter  may  be  used  to  deliver  direct  current  to 
an  Edison  three-wire  system  giving  every  advantage  ordinarily 
obtained  by  the  use  of  two  direct-current  generators  connected  in 
series.  For  such  service  the  third  wire  must  be  connected  to  the 
neutral  point  of  the  transformer  group  in  such  a  manner  that  the 
current  in  the  neutral  will  flow  equally  in  both  directions  through 
the  transformer  windings  and,  therefore,  will  not  change  the  effective 
magnetic  flux  in  the  core  of  the  transformer.  When  three-phase 
distribution  is  used  the  direct  current  neutral  is  brought  out  from 
the  common  junction  point  of  the  inter-connected  Y  secondaries. 
With  two-phase  distribution  the  direct  current  neutral  can  be 
connected  to  the  middle  point  of  the  secondary  coils  of  the  trans- 
former. 

The  connections  of  a  three-phase  rotary  converter  for  sup- 
plying direct  current  to  an  Edison  three-wire  system  are  shown  in 


356 


ALTERNATING-CURRENT  MACHINERY 


Fig.  312.     Connections  for  a  Three-Phase  Rotary 
Converter  on  Edison  Three-Wire  System 


Fig.  312.  The  three  primaries  A,  B,  and  C  of  the  step-down  trans- 
formers for  supplying  alternating  currents  to  the  rotary  converter 

are  either  Y-  or  A-connected 
to  the  high  voltage  supply 
mains.  The  three  secondaries 
a,  b,  and  c  are  Y-connected  to 
the  collector  rings  r' ,  r",  r'"y 
of  the  rotary  converter.  The 
two  outside  direct-current 
mains  1  and  3  are  connected 
to  the  direct-current  brushes 
of  the  converter  as  shown, 

and  the  middle  main  (neutral  main)  2  is  connected  to  the 
common  junction  N,  or  neutral  point,  of  the  Y-connected  sec- 
ondaries a,  b,  and  c.  With  these  connections,  the  voltage  be- 
tween mains  1  and  2  is  a  steady  direct-current  voltage,  as  is  also 
the  voltage  between  mains  2  and  3,  and  each  of  these  voltages  is 
equal  to  half  the  voltage  between  mains  1  and  3.  When  the  number 
of  lamps  connected  between  mains  1  and  2  is  different  from  the 
number  connected  between  mains  2  and  3,  or  vice  versd,  the  neutral 
main  must  carry  a  direct  current  equal  to  the  difference  of  the 
direct  currents  in  the  mains  1  and  3.  This  direct  current  in  the 
neutral  main  2  is  actually  supplied  through  the  secondary  coils  a,  b, 
and  c  from  the  collector  rings.  The  rotary  converter  may  also  be 


Fig.  313.     Connections  for  D.  C.  Generator  on  Edison  Three-Wire  System 
with  Two-Phase  Rotary  Converter  Connected  as  a  Balancer 

used  to  take  the  place  of  two  machines,  a  generator  and  a  motor, 
used  in  combination  to  form  a  "balancer"  for  keeping  the  voltages 
on  the  two  sides  of  an  Edison  three-wire  system  the  same. 

Fig.  313  shows  an  ordinary  direct-current  generator  G,  sup- 


ALTERNATING-CURRENT  MACHINERY  357 

plying  current  to  the  outside  mains  1  and  3,  of  an  Edison  three-wire 
system,  and  a  two-phase  rotary  converter  R,  connected  as  a  "bal- 
ancer" to  supply  the  necessary  direct  current  to  the  middle  main  2. 
The  armature  only  of  the  rotary  converter  is  shown  in  the  figure, 
its  direct-current  brushes  being  connected  to  the  outside  mains  1 
and  3.  One  pair  of  opposite  collector  rings  of  the  converter  R  is 
connected  to  an  inductance  coil  a  a,  wound  on  an  iron  core,  and  the 
other  pair  is  connected  to  an  inductance  coil  b  b.  The  middle  points 
of  these  two  inductance  coils  are  connected  together  and  to  the  mid- 
dle or  neutral  main  2  of  the  three-wire  system. 

Six=Phase  Converter.  The  large  rating  of  a  six-phase  converter 
as  shown  in  Table  X,  together  with  other  advantages  enumerated 
below,  make  this  machine  the  standard  converter  for  large  outputs. 
The  advantages  of  the  six-phase  converter  are  as  follows: 

(a)  The  high  rating,  namely,  1.92  times  the  rating  of  the  same  machine 
as  a  direct-current  generator,  or  1.45  times  the  rating  of  the  same  machine  as 
a  three-phase  converter,  means  that  the  machine  may  be  smaller  and,  there- 
fore, cheaper  for  a  given"  output.     The  high  rating  of  the  six-phase  converter 
is  due  to  the  fact  that  its  armature  winding  is  tapped  at  six  points  (for  a  two- 
pole  machine),  and  that  alternating  currents  enter  the  armature  winding  at 
six  points,  so  that  the  length  and  the  resistance  of  the  paths  from  collector 
rings  to  commutator  are  less,  and  the  heating  of  the  armature  windings  is 
less  than  it  is  in  a  three-phase  converter,  for  example,  for  the  same  direct- 
current  output. 

(b)  The  six-phase  converter  runs  more  stably  than  a  converter  having 
a  smaller  number  of  collector  rings,  and  has  less  tendency  to  hunt. 

(c)  The  magnetizing  actions  of  the  alternating  and  direct  currents  in 
the  armature  are  more  nearly  balanced  in  the  six-phase  converter  than  in  a 
converter  having  a  fewer  number  of  collector  -rings,  and  commutation  is  freer 
from  sparking  and  flashing. 

In  spite  of  the  above  advantages,  the  greater  complication  of 
the  transformer  and  collector  connections  outweighs  them  in  the 
smaller  sizes.  Six-phase  converters,  therefore,  are  rarely  built  in 
units  of  less  than  500  kilo  volt  amperes.  Above  this  output  the  six- 
phase  is  usually  preferable  to  the  three-phase  machine. 

Transformer  Connections  for  Rotary  Converters.  Figs.  314  to 
321  show  the  connections  which  are  commonly  employed  between 
transformers  and  rotary  converters.  The  circular  spiral  winding 
at  the  bottom  of  each  figure  represents  the  armature  winding  of  a 
bipolar  converter,  the  collector  rings  being  omitted  to  avoid  con- 
fusing the  diagram.  Figs.  314,  316,  318,  and  320  show  the  two-phase 


358 


ALTERNATING-CURRENT  MACHINERY 


and  three-phase  connections,  while  figures  315,  317,  319,  and  321  are 
for  six-phase  connection. 


I   Phase  A  I     I  Phase  B 

( oooQOo 


Two-Phase  D/ametr/cal 


Fig.  314.     Transformer  Connections'for 
Two-Phase  Rotary  Converter 


Six-f/jase  Diatnetrical 


Fig.  315.     Transformer  Connections  for  Six- 
Phase  Rotary  Converter 


Each  pair  of  figures,  314  and  315,  316  and  317,  318  and  319, 
320  and  321  are  closely  related.  Thus  in  Figs.  314  and  315,  the 
two  terminals  of  each  transformer  secondary  coil  are  joined  to  the 
armature  winding  of  the  converter  at  points  180  degrees  apart.  Such 
connection  is  called  the  diametrical  connection  and  it  is  possible  when 
the  rotary  has  an  even  number  of  rings.  In  any  diametrical  connec- 
tion of  the  secondaries  of  the  step-down  transformers  (one  trans- 


SGWTOTW 


Three  -Phase  A  Con  nee  t/bn> 

Fig.  316.     Transformer  A  Connections 
for  Three-Phase  Rotary  Converter 


x-f-^hctse  Double  A  Connect /on 


Fig.  317.     Transformer  Double  A  Connec- 
tions for  Six-Phase  Rotary  Converter 


former  for  a  two-ring  converter,  two  transformers  for  a  four-ring 
converter,  and  three  transformers  for  a  six-ring  converter),  the  voltage 


ALTERNATING-CURRENT  MACHINERY 


359 


in  each  secondary  coil  is  the  same  in  value,  no  matter  how  many  rings 
the  converter  may  have. 


>eA|    \PhaseB\    |/°/>aseCi 

UmJ  (smu 


Fig.    318.     Transformer  Y  Connec- 
tions for  Three-Phase  Rotaryj 
Converter 


S/x_/=5»7as«  Double  ^Connection 


Fig.   319.     Transformer  Double  Y  Connec- 
tions for  Six-Phase  Rotary  Converter 


The  diametrical  connection  of  the  transformer  secondaries  to 
the  converter  rings  is  simpler  than  the  connections  shown  in  Figs. 
317,  319,  and  321,  inasmuch  as  the  diametrical  connection  requires 
only  one  secondary  coil  on  each  of  the  step-down  transformers  and, 
therefore,  but  two  secondary  leads  are  brought  out  from  each  trans- 
former; whereas  the  connections  shown  in  Figs.  317,  319,  and  321 
require  two  secondaries  on  each  trans- 
former and,  therefore,  four  secondary 
leads  from  each  transformer.  The  switch- 
ing arrangements  for  the  diametrical 
connection  are,  therefore,  simpler  than 
they  are  for  the  connections  shown  in 
Figs.  317,  319,  and  321. 

Fig.  316  shows  three  step-down 
transformers  receiving  three-phase  cur- 
rents and  delivering  three-phase  currents 
to  a  rotary  converter,  the  secondaries 
being  connected  to  the  collector  rings. 
Fig.  318  shows  the  same  arrangement 
except  that  the  secondaries  are  Y-con- 
nected  to  the  collector  rings.  Fig.  320  shows  a  Scott*  trans- 


Three-Pha.se  ^Connect/on 

Fig.  320.  Transformer  T  Connec- 
tions for  Three-Phase  Rotary 
Converter 


*The  principle  of  phase  transformation  is  explained  in  detail  on  page  257. 


360 


ALTERNATING-CURRENT  MACHINERY 


former  receiving  currents  from  two-phase  supply  mains  and  deliver- 
ing three-phase  currents  to  a  three-ring  converter.  This  Scott 
transformer  arrangement  is  often  called  the  T  connection,  and  it 
may  be  adapted  with  slight  modification  to  three-phase  supply,  that 
is,  to  transform  three-phase  alternating  currents  into  two-phase 
alternating  currents. 

For  three-phase  rotary  converters,  the  transformers  should 
preferably  be  connected  in  A,  as  this  permits  the  system  to  be  oper- 
ated with  only  two  transformers,  in  case  the  third  has  to  be  cut  out 

of  the  circuit  temporarily  for  repairs. 
The  T  connection  as  shown  in 
Figs.  320  and  321  requires  only  two 
transformers,  and  it  can  be  used  to 
change  from  either  two-phase  or 
three-phase  to  three-phase,  as  shown 
in  Fig.  320,  or  from  two-phase  or 
three-phase  to  six-phase,  as  shown 
in  Fig.  321.  The  Y  or  T  arrange- 
ment of  transformers  is  particularly 
advantageous  where  a  rotary  is  to  be 
used  to  supply  direct  current  to  an 
Edison  three-wire  system.  Of  course, 
two  converters  can  be  used,  one  on 
each  branch  of  the  three-wire  system,  and  this  is  the  preferable 
method  where  the  branches  are  liable  to  be  greatly  unbalanced. 
With  the  Y  or  T  connection  of  transformers  a  single  converter 
can  be  connected  across  the  outside  wires  of  the  three-wire  system, 
the  neutral  wire  can  then  be  joined  to  the  neutral  point  of  the  Y 
connection,  or  to  a  tap,  in  one  of  the  transformer  windings,  which 
tap  corresponds  to  the  neutral  point  of  the  Y  connection. 

TESTING  ROTARY  CONVERTERS 

Standard  Tests.  For  a  rotary  converter,  the  saturation  and  sore 
loss  curves  are  obtained  in  the  same  manner  as  in  the  case  of  an 
alternating-current  generator,  with  the  exception  that  the  direct- 
current  voltage  is  also  recorded.  The  phase  characteristic  is  deter- 
mined and  the  pulsation  test  is  made  in  the  same  manner  as  for  a 
synchronous  motor,  except  that  the  direct-current  voltage  should  be 


S/'x-&hctse  Double  T^Connectfon 
Fig.  321.    Transformer  Double  T  Con- 
nections for  Six-Phase  Rotary 
Converter 


ALTERNATING-CURRENT  MACHINERY 


361 


recorded.  The  machine,  of  course,  is  run  as  a  synchronous  motor, 
being  supplied  with  alternating  current  through  its  collecting  rings. 
The  pulsation  test  should  be  made  with  the  converter  self-excited. 

Heat  Run.  To  make  a  heat  run  on  a  rotary  converter  it  may 
be  run  either  as  a  synchronous  motor  taking  alternating  currents 
through  its  collector  rings,  or  as  a  direct-current  motor  supplied 
with  direct  current  through  its  brushes  and  commutator.  If  driven 
as  a  synchronous  alternating-current  motor,  the  full  load  output  is 
taken  from  the  commutator  end  of  its  armature,  or  vice  versa,  and 
is  delivered  to  a  water  rheostat  or  other  receiver.  During  the  run 
the  following  readings  should  be  recorded  every  half  hour: 


Volts 
D.C. 

Volts 
A.  C. 

Amperes 
D.  C. 

Amperes 
A.  C. 

Amperes 
Field 

Volts 
Field 

Speed 

Room 
Temp. 

When  the  rotary  converter  has  reached  a  constant  temperature, 
after  running  continuously  a  number  of  hours  under  rated  full-load 
conditions,  the  machine  may  be  shut  down,  and  thermometers 
quickly  applied  to  measure  the  temperature  of  the  following  parts: 


Field  spools 
Pole  tip,  leading 
Pole  tip,  trailing 
Frame 
Bearings 
Room 


Armature  laminations 
Armature  ventilating  ducts 
Armature  coils  front  end 
Armature  coils  back  end 
Armature  binding  wires 
Commutator 
Collector  rings 

The  condition  of  constant  temperature  is  indicated  when  the 
voltage  applied  to  the  terminals  of  the  field  winding  in  order  to 
keep  the  current  in  the  field  winding  constant,  no  longer  increases. 
In  other  words,  the  resistance  of  the  field  winding  increases  for  a 
time  due  to  increasing  temperature  of  the  winding,  and  it  takes  an 
increasing  voltage  at  the  field  terminals  to  maintain  the  field  cur- 
rent constant.  But  when  the  temperature  of  the  field  windings 
becomes  constant,  the  voltage  required  at  the  field  terminals  no 
longer  increases. 

Motor-Generator  Method.  If  two  similar  rotary  converters  are 
at  hand,  the  heat  test  may  be  made  by  the  "motor-generator"  method, 


362 


ALTERNATING-CURRENT  MACHINERY 


in  a  manner  somewhat  similar  to  the  method  described  under  trans- 
formers. This  method  applied  to  rotary  converters  involves  running 
one  machine  as  an  inverted  rotary  taking  power  through  its  brushes 
and  commutator  from  direct-current  mains.  The  second  machine 
is  made  to  run  as  a  regular  rotary  converter,  taking  its  power  from 
the  collector  rings  (that  is,  alternating-current  side)  of  machine  No. 
1.  Machine  No.  2  then  delivers  its  power  in  the  form  of  direct  cur- 
rent from  its  commutator  back  to  the  direct-current  supply  mains, 
or  to  the  commutator  of  machine  No.  1.  Machines  Nos.  1  and  2  are 
now  said  to  be  "tied  together  in  multiple  on  both  the  direct-current 


Fig.  322.     Wiring  Diagram  for  Motor-Generator  Method  of  Testing  Rotary  Converters 

and  the  alternating-current  sides."  If  the  machines  are  similar  in 
every  respect,  no  appreciable  current  will  flow  between  the  two  under 
these  circumstances.  Each  will  run  from  the  direct-current  side, 
the  two  together  taking  only  enough  power  from  the  direct-current 
mains  to  supply  the  no-load  losses  in  both  machines.  If,  however, 
an  auxiliary  electromotive  force  is  applied  in  the  circuit  between  the 
machines  on  the  alternating-current  side,  it  will  immediately  cause 
current  to  flow  between  the  machines  precisely  as  in  the  case  of  the 
transformer  test  above  referred  to. 

Fig.  322  is  a  diagram  of  the  electrical  connections  for  this  method 
of  running.  R1  and  R2  are  the  two  three-phase  rotaries.  They  are 
shown  connected  together  electrically  on  the  alternating-current 


ALTERNATING-CURRENT  MACHINERY  363 

side  (that  is,  the  collector  rings  of  Ri  are  connected  to  the  corre- 
sponding collector  rings  of  R2)  through  an  induction  regulator  R. 
This  regulator  supplies  to  the  circuit  of  each  phase  the  auxiliary 
electromotive  force  necessary  to  cause  the  current  to  flow  between 
the  two  machines.  The  electromotive  forces  supplied  by  the  regu- 
lator are  adjustable.  When  the  machines  are  first  thrown  together 
the  regulator  is  in  such  a  position  that  its  electromotive  forces  are. 
zero. 

On  the  direct-current  side  (commutator  ends)  both  machines 
are  shown  electrically  connected  to  the  direct-current  supply  mains. 
The  method  of  procedure  in  starting  the  two  machines  is  as  follows: 

First,  RI  is  started  as  a  direct-current  motor  from  the  direct- 
current  side  (commutator).  Its  field  current  is  adjusted  until  it 
runs  at  the  right  speed  when  the  normal  voltage  is  applied  to  its 
armature  terminals;  Ri  is  at  this  time  entirely  disconnected  from 
R2.  Next  R%  is  started  in  the  same  manner.  Then  Ri  and  R%  are 
synchronized  and  connected  together,  as  shown  in  the  diagram,  by 
closing  switches  Si,  S2,  and  Ss  on  the  alternating-current  side.* 

The  most  convenient  way  to  synchronize  the  machines  for  test- 
ing is  to  connect  up  a  series  of  incandescent  lamps  across  each  of  the 
switches  Si,  S2,  and  $3.  The  number  of  incandescent  lamps  in  each 
series  should  be  such  that  they  can  stand  twice  the  normal  alternating 
voltage  of  each  machine. 

The  field  current  of  R2  is  varied  until  it  is  almost  in  synchro- 
nism with  Ri,  as  shown  by  the  slow  pulsations  of  the  synchronizing 
lamps.  When  the  lamps  are  dark,  there  is  zero  voltage  across  the 
terminals  of  the  switches,  and  then  they  can  be  safely  closed.  If 
the  lamps  connected  across  the  switches  do  not  all  go  out  at  the 
same  instant,  the  machines  cannot  be  synchronized.  In  this  case 
the  direction  of  rotation  of  one  of  the  machines  must  be  reversed, 
or  two  of  the  leads  on  the  alternating-current  side  of  one  of  the 
machines  must  be  interchanged. 

Up  to  this  time  the  two  machines  have  been  running  inde- 
pendently of  each  other.  Each  has  taken  sufficient  current  from 
the  mains  to  supply  the  losses  occurring  in  it  while  running  un- 
loaded. When  the  two  machines  are  connected  together  on  the 


*The  apparatus  for  determining  when  the  two  machines  are  in  synchronism  is  not  shown 
in  the  figure. 


364  ALTERNATING-CURRENT  MACHINERY 

alternating-current  side  by  closing  the  switches  Si,  S2,  and  S3,  they 
run  in  synchronism.  If  the  regulator  R  is  in  its  zero  position,  no 
appreciable  current  will  flow  between  the  two  machines.  As  the 
voltage  of  R  is  increased,  however,  a  current  will  circulate  between 
the  two  machines.  If  the  voltage  of  R  is  in  such  a  direction  that 
V2  is  greater  than  V\,  then  R2  will  run  as  a  regular  rotary  converter, 
and  Ei  as  an  inverted  rotary.  The  power  will  pass  from  RI  to  R2 
on  the  alternating-current  side,  and  from  R2  to  RI  on  the  direct- 
current  side.  RI  will  run  as  an  inverted  rotary,  driving  R2  on  the 
alternating-current  side,  and  ^2  will  run  as  an  ordinary  rotary  con- 
verter driving  RI  on  the  direct-current  side. 

By  sufficiently  increasing  the  voltage  of  R  we  can  cause  full- 
load  or  even  over-load  conditions  to  obtain.  The  direct-current 
mains  deliver  merely  the  power  to  supply  the  losses  in  the  entire 
system.  Ammeter  A  indicates  the  current  supplied  to  overcome 
the  losses  in  the  system.  Under  these  circumstances,  RI  will  have 
a  load  slightly  greater  than  R2.  Ammeter  Ai  should  give  a  reading 
equal  to  the  sum  of  the  readings  of  the  two  ammeters  A  and  A2. 

The  chief  advantage  of  this  method .  is  that  full-load  condi- 
tions are  obtained  without  actually  supplying  full-load  current  from 
the  direct-current  supply  mains.  The  mains  supply  simply  the 
power  losses  in  both  machines.  Another  great  advantage  is  the 
ease  with  which  the  load  can  be  controlled.  By  this  method,  more- 
over, two  machines  can  be  tested  more  easily  than  a  single  machine 
by  the  ordinary  method.  If  it  is  desired  to  test  one  machine  only, 
the  load  may  be  put  on  in  the  same  way  by  connecting  it  up  with  a 
second  machine.  It  is  not  necessary  in  this  case  that  the  second 
machine  be  of  the  same  size  as  the  first.  It  is  only  necessary  that 
it  be  of  the  same  voltage  and  frequency,  and  that  it  can  carry  with- 
out excessive  heating  the  full-load  current  of  the  machine  to  be 
tested.  The  current,  during  the  run,  is  adjusted  to  the  normal  full- 
load  value  of  the  machine  to  be  tested. 


MOTOR=QENERATORS 

The  last  method  of  obtaining  direct  current  from  alternating- 
current  circuits  is  by  a  motor-generator,  which  consists  of  an  alter- 
nating-current motor  driving  one  or  more  direct-current  generators. 


ALTERNATING-CURRENT  MACHINERY  365 

The  motor  may  be  of  the  induction  or  of  the  synchronous  type. 
In  either  case  the  motor  is  usually  mounted  on  the  same  base  with, 
and  mechanically  coupled  to,  the  generator.  The  smaller  sets  up  to 
about  150  kilowatts  are  usually  driven  by  induction  motors  for  25, 
40,  or  60  cycles,  and  are  customarily  used  for  charging  storage  bat- 
teries or  for  furnishing  the  exciting  current  for  the  fields  of  alternators 
or  synchronous  motors.  The  larger  sets,  from  200  kilowatts  upwards, 
are  used  for  the  supply  of  lighting,  railway,  or  power  systems,  and 
are  usually  driven  by  synchronous  motors,  which  have  the  advan- 
tage of  permitting  the  control  of  the  power  factor  of  the  circuit  by 
varying  the  field  excitation,  but  have  the  disadvantage  of  requiring 
a  direct-current  field  excitation.  Synchronous  motors  have  an 
advantage  over  induction  motors  in  that  large  sizes  may  be  wound 
for  11,000  or  13,200  volts,  whereas  induction  motors  of  the  same  size 
are  limited  to  2,080  or  6,000  volts. 

Comparison  with  the  Rotary  Converter.  A  motor-generator 
employing  a  synchronous  motor  does  not  seem  to  possess  any  essential 
advantages  over  the  rotary  converter,  except  in  some  cases  where 
independent  control  of  the  direct-current  voltage  is  desired.  The 
use  of  the  synchronous  motor  does  not  remove  the  objections  to  the 
rotary  converter  which  are  based  on  the  fact  that  it  is  a  synchronous 
machine. 

A  motor-generator  employing  an  induction  motor  has  the  ad- 
vantage of  using  induction  instead  of  synchronous  apparatus,  thereby 
securing  many  of  the  advantages  summarized  later  in  the  comparison 
between  synchronous  and  induction  motors,  page  401. 

Circuits  which  are  supplied  by  alternating-current  generators 
the  speed  of  which  has  rapid  and  periodic  fluctuations,  or  in  which 
for  any  other  reason  the  conditions  are  such  as  to  cause  "hunting" 
in  synchronous  machines,  may,  however,  satisfactorily  supply 
an  induction  motor  driving  a  generator.  The  various  characteristics 
of  the  induction  motor  under  emergency  conditions,  such  as  a  sudden 
overload,  momentary  interruption,  or  lowering  of  the  voltage  of  the 
supply  circuit,  may  cause  little  or  no  inconvenience  if  the  induction 
motor  is  used,  whereas  it  might  cause  serious  interruption  to  a  rotary 
converter  or  a  synchronous  motor.  The  induction  motor  driving  a 
generator  is  also  to  be  preferred  where  the  size  of  the  units  is  quite 
small  and  the  attendance  is  unskilled.  It  is  also  preferable  for  single- 


366  ALTERNATING-CURRENT  MACHINERY 

phase  work.  The  armature  for  the  induction  motor  should  in  general 
be  of  the  squirrel-cage  type  as  the  required  starting  torque  does 
not  exceed  20  per  cent  of  full-load  torque,  and  the  required  starting 
torque  can  be  obtained  with  about  full-load  current. 

The  rotary  converter,  like  the  synchronous  motor,  is  not  suit- 
able for  general  distribution  in  small  units.  It  has  the  advantage 
over  the  motor-generator  in  point  of  cost,  there  being  but  one  ma- 
chine instead  of  two;  in  point  of  efficiency,  there  being  the  loss  in 
one  machine  instead  of  two;  and  in  its  effect  upon  the  voltage  of 
the  transmission  system  as  a  whole,  as  it  may  be  compounded  to 
overcome  the  drop  which  would  otherwise  occur  in  generator  and 
transmission  circuit. 

On  the  other  hand,  the  electromotive  force  of  the  direct  current 
delivered  by  the  converter  has  a  more  or  less  fixed  relation  to  the 
electromotive  force  of  the  alternating  currents  supplied;  whereas 
the  electromotive  force  of  a  motor-driven  generator  is  independent 
of  the  electromotive  force  of  the  supply  circuit,  and  it  may  be  ad- 
justed or  compounded  as  desired.  In  practice,  however,  it  is 
found  that  the  voltage  delivered  by  a  rotary  converter  can  be 
satisfactorily  adjusted  and  controlled  by  regulating  devices  or  by 
compounding,  so  that  usually  the  close  relation  between  the  electro- 
motive forces  at  the  two  ends  of  the  converter  is  not  disadvan- 
tageous, provided  the  electromotive  force  of  the  supply  circuit  is 
reasonably  constant.-  This  statement  applies  to  those  cases  where 
a  practically  constant  direct-current  voltage  is  desired.  There  are, 
of  course,  special  cases,  in  which  the  voltage  is  to  be  adjusted  over 
a  wide  range,  or  where,  for  other  reasons,  the  motor-generator  is 
to  be  preferred.  In  many  cases  the  motor  may  be  operated  with- 
out requiring  step-down  transformers,  whereas  they  would  be  neces- 
sary with  rotary  converters. 

Uses  of  Motor=Qenerators.  Motor-generator  sets  are  often 
used  in  central  stations  in  connection,  with  arc-lighting  service. 
In  these  cases  alternating-current  motors  are  used  for  driving  arc- 
lighting  generators,  as  in  the  large  plant  of  the  Buffalo  General 
Electric  Company.  They  are  also  used  in  some  places  for  supplying 
three-wire  direct-current  lighting  circuits  from  alternating-current 
mains. 

Motor-generators  are  often  used  in  central  stations  for  fur- 


ALTERNATING-CURRENT  MACHINERY 

nishing  direct  current  to  the  field-magnet  windings  of  alternators, 
the  direct-current  generator  part  of  the  set  acting  as  the  exciter 
for  one  or  more  large  alternators.  The  alternating  current  supplied 
to  the  (synchronous  or  induction)  motor  end  of  the  motor-generator 
set  is  taken  from  tlie  main  station  bus  bars,  and  may  or  may  not  be 
stepped-down  by  means  of  reducing  transformers. 

Examples  of  this  use  of  motor-generator  sets  are  to  be  ftyund 
in  the  immense  power  stations  of  the  Interborough  Rapid  Transit 
Company,  and  the  New  York  Edison  Company,  of  New  York  City, 
and  in  many  other  places. 

Another  use  for  motor-generators  is  as  frequency  changers 
(or  converters).  In  such  cases  they  are  used  for  transforming  an 


Fig.  323.     General  Electric  Motor-Generator  Set  with  D.  C.  Exciter 

alternating  current  of  one  frequency  to  another  alternating  current 
of  higher  or  lower  frequency,  usually  the  former.  The  current 
may  be  transformed  from  three-phase  to  two-phase,  or  vice  versd, 
at  the  same  time.  Thus  it  might  be  required  to  supply  a  two-phase, 
60-cycle  lighting  system  from  a  25-cycle,  three-phase  transmission 
system,  with  or  without  change  of  voltage. 

Types.  A  motor-generator  set  built  by  the  General  Electric 
Company  is  shown  in  Fig.  323.  It  consists  of  a  combination  of  three 
distinct  machines,  all  mechanically  coupled  to"  the  same  shaft,  viz, 
a  three-phase  synchronous  motor  of  the  revolving-field  type;  a 
three-phase  alternator  also  of  the  revolving-field  type;  and  a  direct- 


368  ALTERNATING-CURRENT  MACHINERY 

current  generator  acting  as  an  exciter  for  the  fields  of  both  the  alter- 
nator and  the  synchronous  motor. 


Summary  of  Data  for  Fig.  323 

Synchronous  Motor  Alternator 

8  12 

275         250 
570        570 
38         57 


Exciter 
10 
10 
570 


No.  of  poles 

Rated  output  (kw.) 

Speed.. 

Frequency 

Volts....  60 

This  particular  motor-generator  set  is  designed  to  be  used  as 
a  frequency-changer  to  change  from  38  to  57  cycles  per  second. 


Fig.  324.     General  Electric  Motor-Generator  Set — Induction  Motor  Driving 
Compound-Wound  D.  C.  Generator 

The  three-phase  alternating  currents  having  a  frequency  of  38 
cycles  per  second  are  led  into  the  stationary  armature  terminals  of 
the  synchronous  motor  (shown  next  to  the  pulley  in  Fig.  323). 
The  motor  having  eight  field  poles,  its  synchronous  speed  is  570 
r.  p.  m.;  and  since  both  the  alternator  and  the  exciter  are  coupled  to 
the  same  shaft,  all  the  machines  run  at  570  r.  p.  m. 

The  alternator  having  12  field  poles,  the  frequency  of  its  in- 
duced electromotive  forces  is  57  cycles  per  second.     Hence,  the 


ALTERNATING-CURRENT  MACHINERY 


369 


three-phase  alternating  electromotive  forces  induced  in  the  station- 
ary armature  of  the  alternator  have  a  frequency  50  per  cent  higher 


than  those  of  the  alternating  currents  which  are  supplied  to  the 
driving  synchronous  motor. 


370  ALTERNATING-CURRENT  MACHINERY 

Fig.  324  is  a  view  of  a  motor-generator  set  built  by  the  General 
Electric  Company.  It  consists  of  a  compound-wound  direct-current 
generator  driven  by  an  induction  motor,  both  machines  being 
mounted  on  the  same  bed-plate  and  having  a  common  shaft. 

Fig.  325  shows  two  motor-generator  sets,  each  consisting  of 
two  Brush  arc-light  dynamos  directly  coupled  to  an  induction 
motor.  In  some  of  the  stations  of  the  Brooklyn  Edison  Company 
Brush  arc-light  dynamos  are  driven  in  pairs  by  being  directly  coupled 
to  three-phase,  25-cycle  synchronous  motors.  The  constancy  of  the 
speed  of  synchronous  motors  makes  them  especially  well  suited  for 
driving  electric  generators. 


ALTERNATING-CURRENT 
MACHINERY 


PART  VI 
INDUCTION  MOTOR 

Constructive  Elements.  It  has  already  been  pointed  out  that 
the  successful  use  of  alternating  current  for  power  purposes  depends 
largely  upon  the  use  of  the  induction  motor  supplied  with  polyphase 
currents.  This  machine  consists  of  a  primary  member  and  a  second- 
ary member,  each  with  a  winding  of  bars  or  wire.  The  primary 


Fig.  326.     Diagram  of  Squirrel-Cage 
Type  of  Rotor 


Fig.  327.     Diagram  Showing  Slots  in 
Stator  of  Induction  Motor 


member  is  usually  stationary,  and  is  often  called  the  stator.  The 
secondary  member  is  usually  the  rotating  member,  and  is  often  called 
the  rotor.  Fig.  326  shows  a  rotor  of  the  squirrel-cage  type.  It  con- 
sists of  a  drum  A  built  up  of  thin  circular  sheet-iron  disks.  Near  the 
periphery  of  this  drum  are  a  number  of  holes  parallel  to  the  axis  of 
the  drum.  In  these  holes  heavy  copper  rods  b  are  placed,  and  the 
projecting  ends  of  these  rods  are  screwed  and  soldered  to  massive 
copper  rings  r,  one  at  each  end  of  the  drum. 

The  stator  is  a  laminated  iron  ring  FF,  Fig.  327,  closely  sur- 
rounding the  rotor.  This  ring  is  slotted  on  its  inner  face,  as  shown; 
windings  are  arranged  in  these  slots,  and  these  windings  receive 

Copyright,  1912,  by  American  School  of  Correspondence. 


372  ALTERNATING-CURRENT  MACHINERY 


current  from  polyphase  supply  mains.     These  polyphase  currents 
produce  in  the  stator  iron  a  rotating  state  of  magnetism,  the  action 


Fig.  328.     Squirrel-Cage  Rotor  Mounted 
in  Ordinary  Four-Pole  Field 


Fig.  329.     Diagram  of  Four-Pole  Two- 
Phase  Induction  Motor 


of  which  on  the  rotor  is  the  same  as  the  action  of  an  ordinary  field 
magnet  mechanically  revolved.  Thus  Fig.  328  shows  a  squirrel- 
cage  rotor  A  surrounded  by  an  ordinary  field  magnet  revolving  in  the 
direction  of  the  arrows.  This  motion  of  the  field  magnet  induces 
currents  in  the  short-circuited  copper  rods  of  the  rotor;  the  field 
magnet  exerts  a  dragging  force  on  these  currents,  and  causes  the  rotor 

to  revolve.  No  electrical  con- 
nections of  any  kind  are  made 
to  the  rotor. 

Stator  Windings  and  Their 
Action.  The  stator  windings 
are  arranged  in  the  slots  s, 
Fig.  327,  in  a  manner  exactly 
similar  to  the  arrangement  of 
the  windings  of  the  two-phase 
or  three-phase  alternator  arma- 
ture according  as  the  motor  is 
to  be  supplied  with  two-phase 
or  three-phase  currents. 

Two-Phase.  Fig.  329  shows 
an  end  view  of  a  four-pole  two- 
phase  induction  motor.  In 
this  figure,  the  outline  only  of  the  rotor  (or  secondary)  is  shown. 
The  stator  conductors  are  represented  in  section  by  the  small  circles, 
the  slots  being  omitted  for  the  sake  of  clearness,  and  the  end  con- 


Fig.  330.     Diagram  of  One-Half  the  Stator 
Conductors  of  Fig.  329 


ALTERNATING-CURRENT  MACHINERY 


373 


nections  of  half  the  stator  conductors  are  shown  in  Fig.  330.  In 
this  figure  the  straight  radial  lines  represent  the  conductors  which 
lie  in  the  slots  of  the  stator,  and  the  curved  lines  represent  the  end 
connections.  The  stator  conductors  are  arranged  in  two  distinct 


Iron 


Iron 


O  Q  Q  O  O 


ffon 


Up  f~bv*iny  Cur  fen  ts 

Fig.  331.     Magnetic  Action  of  Conductors 
Carrying  Currents  between  Iron  Poles 


Iron 


Oo  wn  Flo  wing  Cure  en  ts 

Fig.  332.     Magnetic  Action  of  Conductors 
Carrying  Currents  between  Iron  Poles 


circuits.  One  of  these  circuits  includes  all  of  the  conductors  marked 
A,  and  this  circuit  receives  current  from  one  phase  of  a  two-phase 
system.  The  other  circuit  includes  all  of  the  conductors  marked  B 
and  this  circuit  receives  current  from  the  other  phase  of  the  two- 
phase  system.  The  terminals  of  the  B  circuit  are  shown  at  t  tf,  Fig. 
330.  The  conductors  which  constitute  one  circuit  are  so  connected 
that  the  current  flows  in  opposite  directions  in  adjacent  groups  of 
conductors  as  indicated  by  the  arrows  in  Fig.  330. 

The  action  of  a  band  of  conductors  between  the  two  masses  of 
iron  is  shown  in  Figs.  331  and  332.  The  small  circles  represent  the 
conductors  in  section;  conductors  carrying  down-flowing  currents 


Fig.  333.    Instantaneous  Effect  of  Alternating        Fig.  334.    Magnetic  Effect  of  Alternating  Cur- 
Current  for  a  Given  Position  of  Rotor  rent  i  of  a  Cycle  Later  than  in  Fig.  333 

are  marked  with  crosses  and  those  carrying  up-flowing  currents  with 
dots.    The  action  of  the  currents  in  these  bands  of  conductors  is  to 


374 


ALTERNATING-CURRENT  MACHINERY 


Fig.  335.     Magnetic  Effect  of  Alternating 

Current  J  of  a  Cycle  Later  Than  in 

Fig.  333 


produce  magnetic  flux  along  the  dotted  lines  in  the  direction  of  the 

arrows. 

The  lines  A'  and  B'  in  the  clock  diagrams  of  Figs.  333,  334, 

and  335  are  supposed  to  rotate,  and  their  projections  on  the  fixed 

line  ef  represent  the  instantane- 
ous values  of  the  alternating  cur- 
rents in  the  A  and  B  conductors, 
respectively.  These  conductors 
are  represented  in  section  by  the 
small  circles.  These  small  cir- 
cles are  marked  with  crosses 
when  they  carry  down-flowing 
currents,  with  dots  when  they 
carry  up-flowing  currents,  and 
they  are  left  blank  when  they 
carry  no  current. 
Fig.  333  shows  the  state  of  affairs  when  the  current  in  conductors 

A  is  a  maximum  and  the  current  in  conductors  B  is  zero,  and  the 

dotted  lines  indicate  the  paths  of  the  magnetic  flux.    This  flux  enters 

the  rotor  from  the  stator  at  the  points  marked  N  and  leaves  the  rotor 

at  the  points  marked  S. 

Fig.  334  shows  the  state  of  affairs,  one-eighth  of  a  cycle  later, 

when  the  current  in  the  B  conductors  has  increased  and  the  current 

in  the  A  conductors  has  de- 
creased to  the  same  value,  so 
that  equal  currents  flow  in  the 
A  and  in  the  B  conductors.  The 
points  N  and  S  have  moved  over 
one-sixteenth  of  the  circumfer- 
ence of  the  stator  ring,  from  the 
positions  they  occupied  in  Fig. 
333. 

Fig.  335  shows  the  state  of 
affairs,  after  another  eighth  of  a 
cycle,  when  the  current  in  the 
B  conductors  has  reached  its 

maximum  value,  and  the  current  in  the  A  conductors  has  dropped 

to  zero.    The  points  N  and  S  have  moved  again  over  one-sixteenth 


A 


Fig.  336.     Complete  Connections  for  Four 

Poles  of  One  Circuit  of  Three-Phase 

Induction  Motor 


ALTERNATING-CURRENT  MACHINERY  375 

of  the  circumference  of  the  stator  ring.  This  motion  of  the  points 
N  and  S  is  continuous,  and  these  points  make  one  complete  revolu- 
tion (in  a  four-pole  motor)  during  two  complete  revolutions  of  the 
vectors  A'  and  B',  that  is,  while  the  alternating  currents  supplied 
to  the  stator  windings  are  passing  through  two  cycles.  In  general: 


n  =  —  (47) 


in  which  n  is  the  revolutions  per  second  of  the  stator-magnetism, 
p  is  the  number  of  poles,  N  and  S,  and  /  is  the  frequency  of 
the  alternating  currents  supplied  to  the  stator  windings. 

Three-Phase.  When  an  induction  motor  is  driven  by  three- 
phase  currents,  the  stator  conductors  are  arranged  in  three  distinct 
circuits  A,  B,  and  C,  which  are  either  A-connected  or  Y-connected 
to  the  supply  mains.  Fig.  336  shows  the  complete  connections,  for 
four  poles,  of  the  A  circuit  with  its  terminals  t 1'.  The  B  and  C 
circuits  are  similarly  connected. 

In  general,  the  g-phase  stator  winding  for  p  poles  has  p  q  equi- 
distant bands  of  conductors.  The  first,  (g-f-l)th,  (2g+l)th,  etc., 
bands  are  connected  in  one  circuit,  so  that  currents  flow  oppositely 
in  adjacent  bands,  and  this  circuit  takes  current  from  one  phase  of 
the  g-phase  system.  The  second,  (g+2)th,  (2g+2)th,  etc.,  bands 
are  similarly  connected  in  another  circuit  and  take  current  from 
the  second  phase  of  the  g-phase  system.  The  third,  (g-f-3)th,  (2g 
+3)th,  etc.,  bands  are  similarly  connected  in  another  circuit  and 
take  current  from  the  third  phase  of  the  g-phase  system;  and  so  on. 

ACTION  OF  INDUCTION  MOTOR 

Many  important  details  of  the  action  of  the  induction  motor 
are  most  easily  explained  by  looking  upon  the  induction  motor  as  a 
rotor  influenced  by  an  ordinary  field  magnet,  mechanically  revolved. 
The  complete  theory  of  the  action  of  the  induction  motor  is,  how- 
ever, similar  to  the  theory  of  the  alternating-current  transformer. 

Torque  and  Speed.  Let  n  be  the  number  of  revolutions  per 
second  of  the  field,  and  n'  the  revolutions  per  second  of  the  rotor. 
When  n—n',  the  rotor  and  field  revolve  at  the  same  speed,  so  that 
their  relative  motion  is  zero;  no  electromotive  force  is  then  induced 
in  the  rotor  conductors  and  no  current  flows  and,  therefore,  the 
revolving  field  exerts  no  torque  upon  the  rotor.  As  the  speed  of  the 


376 


ALTERNATING-CURRENT  MACHINERY 


rotor  decreases,  the  difference  of  the  speeds  of  rotor  and  field  (n—  n'), 
increases  and,  therefore,  the  electromotive  force  induced  in  the  rotor 
conductors,  the  currents  in  the  conductors,  and  the  torque  with 
which  the  field  drags  the  rotor,  all  increase.  If  the  whole  of  the  field 
flux  were  to  pass  into  the  rotor  and  out  again  in  spite  of  the  demag- 
netizing action  of  the  current  in  the  rotor  conductors,  then  the 
torque  would  increase  in  strict  proportion  to  (n—  n').  As  a  matter 
of  fact,  because  of  the  demagnetizing  action  of  the  rotor  currents, 
a  larger  and  larger  portion  of  the  field  flux  passes  through  the  space 
between  the  stator  and  rotor  conductors  as  the  speed  of  the  rotor 
decreases,  and  this  magnetic  leakage  causes  the  torque  to  increase  more 
and  more  slowly  as  (n—nf)  increases.  The  torque  usually  reaches  a 
maximum  value,  and  then  decreases  with  further  increase  of  (n—n'). 
Fig.  337  shows  the  typical  relation  between  torque  and  speed 
of  an  induction  motor.  Ordinates  of  the  curve  represent  torque, 
and  abscissas  measured  from  0  represent  rotor  speeds.  The  rotor 
is  said  to  run  above  synchronism  when  it  is  driven  so  that  nf  is 

greater  than  n.  The  rotor 
never  actually  reaches  syn- 
chronous speed,  but  ap- 
proaches it  very  nearly 
when  the  induction  motor 
is  running  unloaded.  In 
order  to  cause  the  rotor  to 
run  above  synchronism, 
that  is,  to  make  n'  greater 
than  n,  or  in  order  to  cause  the  rotor  to  run  backwards,  that  is, 
in  a  direction  opposite  to  that  of  the  revolving  magnetism  in  the 
stator  iron,  the  rotor  must  be  driven  mechanically  from  an  outside 
source  of  power. 

Starting  Resistance  in  the  Rotor  Windings.  The  speed  of 
the  rotor  for  which  the  maximum  torque  occurs,  depends  upon  the 
resistance  of  the  rotor  windings,  and  it  is  advantageous  under  cer- 
tain conditions  of  operation  to  provide  at  starting  such  resistance 
in  these  windings  as  to  at  once  produce  the  maximum  torque, 
this  resistance  being  cut  out  as  the  motor  approaches  full  speed. 

Efficiency  and  Speed.  For  the  sake  of  simplicity,  let  us  assume 
that  the  only  opposition  to  motion  of  the  revolving  field  magnet 


Fig.  337. 


Graphical  Relation  of  Torque  and  Speed 
of  Induction  Motor 


ALTERNATING-CURRENT  MACHINERY  377 

is  the  reaction  of  the  torque  which  it  exerts  on  the  rotor.  Let  this 
torque  be  represented  by  T.  Then  the  power  expended  in  driving 
the  field  magnet  is  2nnT,  and  the  mechanical  power  delivered  to 
the  rotor  is  2nn'T,  and  this  power  Znn'T  is  available  at  the  pulley 
of  the  motor,  except  for  slight  losses  due  to  friction  in  the  bearings 
and  to  air  friction.  Therefore,  ignoring  friction  losses,  2nnT  is  the 
input  of  power  in  driving  the  revolving  field,  and  2nn'  T  is  the  output 

of  power,  so  that  the  efficiency  of  the  induction  motor  is — . 

n 

This  expression  for  efficiency  ignores  all  the  losses  of  power 
in  the  revolving  field  magnet*  and  the  friction  and  windage  losses 
in  the  rotor,  and  shows  that  the  efficiency  of  an  induction  motor  is 
zero  when  the  rotor  stands  still,  that  it  increases  as  the  rotor  speeds 
up,  and  approaches  100  per  cent,  ignoring  field  losses  and  friction,  as 
the  rotor  speed  approaches  the  speed  of  the  revolving  field.  The 

n' 
ratio  - —  ranges  from  0.85  to  0.95  or  more,  in  commercial  induction 

iL 

motors  under  full  load,  but  the  actual  full  load  efficiencies  of  induc- 
tion motors  range  from  75  per  cent,  or  even  less  for  small  motors, 
to  about  95  per  cent  for  very  large  motors. 

Ratio  of  Mechanical  to  Electrical  Energy  in  Rotor.  The  total 
power  delivered  to  the  rotor  is  equal  to  2nnT  where  n  and  T  have 
the  meanings  above  specified.  That  is,  all  of  the  power  used  to  drive 
the  field  magnet  (ignoring  losses  in  the  field)  is  delivered  to  the  rotor. 
Now,  the  mechanical  power  delivered  to  the  rotor  is  equal  to  2nn'T, 
as  already  explained;  therefore,  the  difference  2nnT—  fl7tn'T\  is 
electrical  power  used  to  force  the  rotor  currents  through  the  rotor 
windings. 

Therefore,  when  the  field  speed  is  n  and  the  rotor  speed  is 
nf,  the  total  power  delivered  to  the  rotor,  the  mechanical  power 
developed  in  turning  the  rotor,  and  the  electrical  power  developed 
in  the  rotor  windings  are  to  each  other  as,  n,  n',  and  (n—n'),  re- 
spectively. 


*In  the  actual  induction  motor,  it  ignores  the  loss  of  power  due  to  the  heating  of  the 
stator  windings  by^  the  supplied  alternating  currents,  and  the  loss  of  power  due  to  core  losses 
in  the  stator  iron. 

fWhen  torque  is  expressed  as   pounds  weight   on  a  lever  arm  of   one  foot  in  length 

the  torque  is  said  to  be  expressed  in  pound-feet,  and  power  in  watts  is  equal  to  — ~T^ 

—  8.52nT  watts,  where  n  is  the  speed  in  revolutions  per  second. 


378  ALTERNATING-CURRENT  MACHINERY 

Ratio  of  Rotor  Voltages  to  Stator  Voltages.  When  the  rotor 
is  wound  with  the  same  number  of  conductors  as  the  stator,  then 
when  the  rotor  is  standing  still,  the  rotating  stator  magnetism  in- 
duces in  the  rotor  windings  electromotive  forces  of  the  same  value 
and  of  the  same  frequency  as  the  electromotive  forces  induced  in 
the  stator  windings  by  this  rotating  stator  magnetism  (neglecting 
magnetic  leakage).  Moreover,  the  electromotive  forces  induced  in 
the  stator  windings  are  very  nearly  equal  and  opposite  to  the  volt- 
ages applied  to  the  stator  windings.  When  the  difference  of  the 
speeds  of  the  rotor  and  the  stator  magnetism  is  (n—nf),  the  electro- 
motive forces  induced  in  the  rotor  windings  are  the  fractional  part, 

I,  of  the  voltages  applied  to  the  stator  windings,  and  the 

frequency  of  the  electromotive  forces  induced  in  the  rotor  windings 

[H  —  nf  ~i 
I     of  the  frequency  of  the  voltages 
//  ~J 

applied  to  the  stator  windings. 

Example.  Let  a  certain  three-phase  induction  motor  having  a  stator 
wound  for  6  poles,  and  taking  three-phase  alternating  currents  at  a  frequency 
of  60  cycles  and  a  voltage  of  220  between  any  two  of  the  three  supply  mains, 
have  a  rotor  furnished  with  the  same  number  of  conductors  as  the  stator. 
Further,  let  the  no-load  speed  of  the  rotor  be  1194  r.p.m.,  and  its  full-load 
speed  be  1,143  r.  p.  m.  Assuming  that  the  magnetic  leakage  is  negligible,  it 
is  required  to  find: 

(a)  The  synchronous  speed. 

(b)  The  electromotive  forces  (three-phase)  induced  in  the  rotor  wind- 
ings at  no-load  and  at  full-load. 

(c)  The  frequency  of  the  electromotive  forces  induced  in  the  rotor 
windings  at  no-load  and  at  full-load. 

Solution:  (a)  The  synchronous  speed  of  the  rotating  magnetism  in 
the  stator  is,  according  to  equation  (47), 

60 
n  =  —  =  20  revolutions  per  second,  or  1,200  r.p.m. 

IF 

(b)  The  electromotive  forces  (three-phase)  induced  in  the  rotor  wind- 
ings at  no-load  are 

-  X  voltage  applied  to  stator 


or 

/  1200  -  1194 
\         1200 


)x220  =  1.10  volts 


ALTERNATING-CURRENT  MACHINERY 


379 


The  voltages  induced  in  the  rotor  windings  at  full-load  are 
/  1200-1143 


1200 


X  220  =  10.45  volts 


It  is  interesting  to  note  that  if  the  slip  of  the  rotor  at  no  load  were  zero, 
or  in  other  words,  if  n'  were  equal  to  n  there  would  be  zero  electromotive 
forces  induced  in.  the  rotor  windings  at  no-load. 

(c)  The  frequency  of  the  electromotive  forces  induced  in  the  rotor 
windings  at  no-load  is 

-  X  frequency  of  stator  voltage 


or 


1200  -1 194  \ 

—  )  X  60  =  0.3  cycles  per  second 


V         1200 

The  frequency  of  the  electromotive  forces  induced  in  the  rotor  wind- 
ings at  full-load  is 


1200 -1 143  \ 
1200         / 


X  60  =  2.85  cycles  per  second 


When  the  rotor  is  at  rest,  the  frequency  of  the  electromotive  forces 
induced  in  the  rotor  windings  is  the  same  as  the  frequency  of  the  stator  volt- 
age, namely,  60  cycles  per  second. 

Efficiency  and  Rotor 
Resistance.  For  a  given 
difference  (n—nf)  between 
field  speed  and  rotor  speed, 
a  definite  electromotive 
force  is  induced  in  the  rotor 
conductors,  and  the  less  the 
rotor  resistance,  the  greater 
the  current  produced  by 
this  electromotive  force,  and 
the  greater  the  torque. 
Therefore,  a  given  induc- 
tion motor  will  develop  its 
full  load  torque  for  a  small 

n' 

value  (n—nf)  or  for  a  large  value  of  — •  (efficiency)  if  its  rotor  resist- 

n 

ance  is  small.    High  efficiency  depends,  therefore,  upon  low  rotor  re- 
sistance. 

The  necessity  of  high  rotor  resistance  to  give  large  torque  at 
starting  has  nothing  to  do  with  the  necessity  of  making  the  rotor 


Fig.  338.     Iron  Stator  Frame  for  Westinghouse 
Induction  Motor 


380 


ALTERNATING-CURRENT  MACHINERY 


resistance  small  in  order  to  secure  full  load  torque  at  as  nearly 
synchronous  speed  as  possible.  These  two  conflicting  conditions 
may  be  realized  in  one  motor  by  an  arrangement  whereby  a  resist- 
ance which  is  in  circuit  with  the  rotor  conductors  at  starting,  may 
be  short-circuited  when  the  motor  nearly  reaches  its  rated  speed. 
Structural  Details  of  a  Typical  Induction  Motor.  Figs.  338  to 
341  show  the  structural  details  of  a  typical  induction  motor  manu- 
factured by  the  Westinghouse  Electric  and  Manufacturing  Company. 
It  has  a  stationary  primary  member  (often  called  the  stator  or  field) 
and  a  rotating  secondary  member  (often  called  the  rotor  or  armature). 

The  primary  member  is 
mounted  in  a  hollow 
cylindrical  frame  of  cast 
iron  shown  in  Fig.  338. 
This  frame  forms  a  base 
for  the  machine,  and  also 
supports  the  two  end- 
brackets  which  carry  the 
self -oiling  bearings.  In- 
side the  frame  are  sev- 
eral lugs  that  support 
the  stator  core  lamina- 
tions far  enough  from 
the  frame  to  leave  space 
between  the  frame  and 
the  core  for  ventilation. 
The  iron  core  of  the  pri- 
mary member  consists  of 
a  ring  built  up  of  sheet- 
steel  stampings  slotted  on  the  inside  to  receive  the  primary  con- 
ductors as ;  shown  in  Fig.  339.  The  laminations  are  assembled, 
clamped,  and  keyed  between  stiff  end  rings  inside  the  lugs  on  the 
frame.  Steel  keys  in  one  or  more  of  the  lugs  prevent  circular  move- 
ment of  the  laminations.  Fig.  339  shows  the  primary  member  com- 
pletely wound. 

The  primary  conductors  are  usually  grouped  in  former-wound 
coils  of  wire  which  are  thoroughly  taped  and  insulated  before  being 
slipped  into  place  in  the  slots  in  the  stator  core.  In  larger  motors 


Fig.  339. 


Primary  Member  of  Westinghouse  Induc- 
tion Motor  Completely  Wound 


ALTERNATING-CURRENT  MACHINERY 


381 


Iron  Rotor  Core  for  Westinghouse  Induc- 
tion Motor 


copper  strap  bent  into  the  proper  form  is  used  instead  of  wire  for 
forming  the  coils. 

The  terminals  of  the 
primary  winding  are  brought 
out,  usually  at  one  side  of 
the  motor,  and  are  clamped 
in  insulated  cleats  or  bush- 
ings. The  leads  which  sup- 
ply alternating  currents  to 
the  motor  are  attached  to 
these  terminals  through  Fig.  340 
suitable  connectors. 

The  iron  core  of  the  secondary  member  (rotor),  shown  in  Fig. 
340,  is  also  built  up  of  ring-shaped  stampings  of  sheet-steel  assembled, 
clamped,  and  keyed  between  stiff  end  plates  on  the  arms  of  the  rotor 
spider.  The  spider  is  pressed  on  the  shaft  and  keyepl.^  Ventilating 
plates  on  the  rotor  cores  of  the  larger  motors  act  like  the  blades  of  a 
fan  and  force  strong  currents  of  ,air  between  the  rotor  end  rings  and 
the  core  and  through  all  the  openings  in  and  around  the  stator  wind- 
ings and  core,  thus  keeping  all  parts  cool.  The  secondary  conductors 
consist  of  rectangular  cop- 
per bars  placed  in  nearly 
closed  slots,  around  the 
periphery  of  the  core.  These 
bars  project  beyond  the 
laminated  core,  and  they 
are  screwed  and  soldered 
at  each  end  to  massive  rings 
of  copper,  thus  forming  a 
short-circuited  secondary 
winding,  as  shown  in  Fig. 
340.  This  type  of  secondary 
member  is  called  a  squirrel- 
cage  rotor.  The  complete 
motor  is  shown  in  Fig.  341 . 

Types  of  Rotors  for  Constant  and  Variable  Speed.  There  are 
three  types  of  secondary  members  used  in  commercial  induction 
motors,  according  to  the  conditions  of  service  to  be  met.  The  start- 


Fig.  341. 


Westinghouse  Induction  Motor  Com- 
pletely Assembled 


382  ALTERNATING-CURRENT  MACHINERY 

ing  and  running  conditions  determine  which  type  to  adopt  in  any 
given  case.    These  types  are  shown  in  Figs,  342,  343,  and  344. 


Fig.  342.     Squirrel-Cage  Type  of  Rqtor 

/Fig.  342  is  a  squirrel-cage  rotor.  Fig.  343  is  a  rotor  wound 
with  insulated  wire,  forming  what  is  called  a  definite*,  or  polar, 
winding.  The  terminals  of  this  winding  are  connected  to  a  start- 
ing resistance  mounted  inside  of  the  rotor.  A  switch  is  arranged  to 
short-circuit  this  starting  resistance,  and  is  operated  while  the  motor 
is  running  by  means  of  a  rod  which  lies  inside  of  the  hollow  shaft  of 


Fig.  343.     Rotor  with  Definite  or  Polar  Winding  with  Starting  Resistance  Inside  Core 

the  rotor.  This  rod  terminates  in  a  small  handle  or  knob  at  the  end 
of  the  rotor  shaft  as  shown  in  Fig.  343.  Fig.  354  shows  a  complete 
three-phase  induction  motor  with  the  knob  and  rod  for  operating 
the  internal  starting  resistance. 

*This  winding  is  identical  with  the  stator  winding  as  described  on  page  374. 


ALTERNATING-CURRENT  MACHINERY  383 

Fig.  344  is  a  rotor  with  a  winding  similar  to  the  winding  of 
Fig.  343,  but  instead  of  connecting  the  terminals  of  the  rotor  winding 
to  an  internal  starting  resistance,  these  terminals  are  brought  out 
to  collector  rings  on  the  end  of  the  shaft  as  shown  in  Fig.  344.  The 
circuits  of  the  rotor  windings  are  completed  through  adjustable 
external  resistances  which  are  connected  to  the  rotor  windings  by 
means  of  brushes  rubbing  on  the  collector  rings.  These  adjustable- 
external  resistances  are  regulated  by  a  cylindrical  switch  or  controller 
similar  in  general  to  the  ordinary  electric  street-car  controller. 
Fig.  345  shows  a  Westinghouse  controller  for  induction  motors  used 
for  cranes,  hoists  and  similar  apparatus. 

The  cylinder  has  a  set  of  contacts  for  making,  breaking,  and 
reversing  the  primary  circuit,  and  another  set  of  contacts  for  control- 


rig.  344.     Rotor  with  Polar  Winding  and*  Collector  Rings 

ling  the  speedcof  the  motor  by  varying  the  resistance  in  the  secondary 
circuits.  The  two  sets  of  contacts  on  two  drums  are  mounted  on 
the  same  shaft  and  all  operations  are  performed  by  moving  one 
controller  handle.  The  number  of  speed  steps  in  each  direction  of 
rotation  may  be  6,  9,  12,  or  15,  according  to  the  capacity  of  the 
controller. 

An  induction  motor  provided  with  a  squirrel-cage  rotor  takes 
excessive  current  from  the  alternating-current  supply  mains  at 
starting.  The  squirrel-cage  rotor  requires  from  three  to  four  times 
full-load  current  to  produce  at  starting  a  torque  equal  to  the  torque 
developed  when  running  at  full-load.  Hence,  when  the  motor  has 
to  start  under  a  heavy  load,  or  where  the  taking  of  excessive  currents 
from  the  supply  mains  will  interfere  with  other  apparatus  supplied 


384          ALTERNATING-CURRENT  MACHINERY 

from  the  same  mains  by  causing  excessive  drop  of  voltage,  the  squirrel- 
cage  type  of  rotor  is  objectionable,  especially  in  large  size  motors. 
On  the  other  hand,  the  extreme  simplicity  of  the  squirrel-cage  rotor 
and  its  ability  to  carry  enormous  currents  without  injury,  largely 
compensate  for  the  above  mentioned  disadvantages.  Its  speed  is 
practically  constant,  varying  only  a  few  per  cent  from  full  load  to  no 
load.  The  operating  characteristics  of  the  squirrel-cage  type  of 
induction  motor  are  such  as  adapt  them  to  a  wide  variety  of  pur- 
poses and  make  them  especially  suitable  for  continuous  constant- 
speed  service. 

An  induction  motor  provided  with  a  rotor  like  that  shown  in 
Fig.  343,  with  an  internal  starting  resistance,  takes  at  starting  only 

about  one  and  one-half  full-load  rated 
current  from  the  supply  mains,  giving  a 
starting  torque  of  about  one  and  one- 
half  full-load  torque.  Such  an  induction 
motor,  therefore,  is  used  only  where  a 
starting  torque  not  greatly  in  excess  of 
full-load  torque  is  required.  The  advan- 
tage of  this  type  of  rotor  is  that  it 
does  not  take  excessive  currents  at  start- 
ing, and  it  will  start,  therefore,  with- 
out producing  excessive  drop  of  electro- 
motive force  in  the  alternating-current 
system  from  which  the  motor  receives  its 
power.  The  starting  resistance  in  motors 
up  to  about  50  horse-power  consists  of 
cast-iron  grids  enclosed  in  a  triangular 
frame  which  is  bolted  to  the  end  plates 
holding  the  rotor  laminations  together. 

Fig.  345.  Westinghouse  Controller     rrn  r     i          «    ,  -,  .  .    ,  .       . 

for  Induction  Motors  1  ne    Wnole  OI  thlS  resistance    IS    in    SCRCS 

with  the  secondary  winding  at  starting. 

As  the  motor  increases  in  speed,  the  resistance  is  short-circuited 
by  sliding  spring  metal  brushes  along  the  inside  surface  of  the 
grids.  The  brushes  are  supported  by  a  metal  sleeve  sliding  on 
the  shaft  which  is  operated  by  a  rod  passing  through  the  end  of 
the  shaft. 

An  induction  motor  having  a  rotor  provided  with  collector 


ALTERNATING-CURRENT  MACHINERY  385 

rings  is  generally  used  for  cranes,  hoists,  elevators,  and  other  work 
where  variable  speed  is  required.  The  starting  resistance  used  in 
the  type  of  rotor  shown  in  Fig.  343  is  designed  to  carry  the  rotor 
current  for  a  short  time  only,  that  is,  during  starting;  if  kept  con- 
tinuously in  circuit  for  the  purpose  of  speed  control,  this  starting 
resistance  would  become  excessively  hot.  For  speed  control,  there- 
fore, an  external  resistance  must  be  used. 

The  range  of  speed  control  possible  in  the  case  of  an  induction 
motor  provided  with  a  rotor  having  collector  rings  connected  to 
external  adjustable  resistances  is  about  the  same  as  the  range  of 
speed  control  obtainable  with  a  shunt-wound  direct-current  motor 
having  a  regulating  rheostat  in  its  armature  circuit. 

Behavior  at  Starting  and  in  Operation.  When  an  induction 
motor  is  running  without  load,  its  speed  is  nearly  equal  to  the  speed 
of  the  rotating  magnetic  field,  namely,  synchronous  speed.  Under 
these  conditions  the  stator  takes  only  sufficient  current  to  force  the 
magnetic  flux  through  the  reluctance  of  the  magnetic  circuit,  and  to 
supply  the  PR  losses  of  the  stator  windings,  the  core  loss,  and  the 
friction  and  windage  loss  of  the  rotor. 

When  the  motor  is  loaded,  its  speed  decreases  in  nearly  direct 
proportion  to  the  load,  from  nearly  synchronous  speed  at  no-load 
to  about  98  per  cent  of  synchronous  speed  in  the  case  of  large 
motors,  and  to  about  92  per  cent  of  synchronous  speed  in  small 
motors  at  full-load.  Therefore,  the  induction  motor  is  practically 
a  constant-speed  motor.  The  decrease  in  speed  expressed  as  a  per- 
centage of  synchronous  speed  is  called  the  slip  of  the  motor.  The 
slip  of  large  motors  is  thus  about  2  per  cent  at  full-load,  and  that  of 
small  motors  is  about  8  per  cent  at  full-load. 

When  an  induction  motor  is  overloaded,  it  takes  excessive  cur- 
rent from  the  supply  mains,  and  its  torque  increases  up  to  a  cer- 
tain value  of  the  slip  (a  definite  value  for  a  given  motor).  When 
loaded  up  to  this  point  the  machine  is  unstable,  and  the  least  ad- 
ditional loading  causes  the  machine  to  "break  down"  or  stop. 

This  maximum  output  which  a  given  motor  can  deliver  is 
usually  about  one  and  one-half  to  two  and  one-half  times  as  great  as 
its  rated  full-load  output.  This  maximum  output  is  proportional 
to  the  square  of  the  electromotive  force  of  the  alternating  currents 
supplied  to  the  motor.  Thus  a  certain  induction  motor  rated  at  220 


386          ALTERNATING-CURRENT  MACHINERY 


volts  has  a  maximum  power  output  of  1 .8  times  its  rated  output.    The 
same  motor  supplied  with  currents  at  200  volts  would  have  a  maxi- 

mum  output  off-  -)*  X  1.8  =  1.49  of  its  rated  output. 

When  an  induction  motor  is  operated  at  slightly  less  than  its 
rated  frequency  but  with  full-rated  voltage,  the  speed  of  the  motor 
will  be  decreased  in  proportion  to  the  frequency,  but  its  power 
output  will  not  be  greatly  affected.  The  efficiency  of  the  motor, 
and  its  rise  of  temperature  under  full-load,  will  be  approximately 


Fig.  346.     Three-Phase  Starting- 
Compensator  Complete/'' 


Fig.  347.     Three-Phase  Starting  Com- 
pensator with  Cover  Removed 


unchanged,  and  the  maximum  power  output  will  be  slightly  increased. 
An  induction  motor  having  a  squirrel-cage  rotor  will  develop 
sufficient  torque  to  start  satisfactorily  with  from  40  per  cent  to  80 
per  cent  of  the  rated  voltage  applied  to  the  primary  member.  There- 
fore, the  current  required  at  starting  may  be  greatly  reduced  by 
supplying  the  primary  member,  at  starting,  with  current  through  a 
step-down  transformer  which  is  designed  to  reduce  the  supply 
voltage  to  40,  50,  60,  or  80  per  cent  of  the  rated  voltage  of  the 
motor,  and  to  multiply  the  delivered  current  in  the  same  ratio.  This 


ALTERNATING-CURRENT  MACHINERY 


387 


step-down  transformer  is  usually  an  autotransformer.  An  auto- 
transformer,  with  its  special  switching  device  for  changing  the  motor 
connections  quickly  from  the  low-starting  voltage  to  the  full- 
running  voltage,  is  called  an  autostarter,  or  a  compensator.  The 
autostarter  may  be  located  at  any  convenient  point,  either  near  the 
motor  or  at  a  distance  from  it. 

Figs.  346  and  347  are  general  views  of  a  three-phase  hand- 
operated  starting  compensator  of  the  wall  type,  as  manufactured 
by  the  General  Electric  Company.  The  compensator  consists  of 
three  core-type  autotransformers,  a  cable  clamp,  and  a  special 
switch  assembled  in  a  metal  case  with  external  handle  and  release 
lever.  In  the  wall-suspension  type  the  switch  is  located  at  the  bot- 
tom, as  seen  in  Fig.  347,  and  is  enclosed  by  an  oil-filled  tank,  Fig. 


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Fig.  348.     Electrical  Connections  for  Three-Phase  Starting  Compensator 

348  is  a  diagram  of  the  electrical  connections.  The  three  coils  of  the 
three-phase  winding  are  connected  in  Y,  the  line  to  the  three  free 
ends  of  the  coil,  and  the  starting  connections  of  the  motor  to  the 
taps.  For  motors  from  5  to  18  horse-power,  these  compensators  are 
provided  with  taps  for  starting  the  motor  at  50,  65,  and  80  per  cent 
of  the  line  voltage,  any  one  of  which  may  be  selected  after  trial  for 
permanent  connection  to  the  switch,  for  starting  according  to  the 
requirements  of  any  individual  case. 

The  shaft  of  the  switch,  as  seen  in  Fig.  347,  extends  through 
the  sides  of  the  compensator  case,  and  is  operated  by  a  lever  at  the 
right,  being  held  in  the  running  position  by  a  lever  at  the  left.  The 
switch,  provided  with  heavy  wiping  contacts,  is  immersed  in  oil 


388  ALTERNATING-CURRENT  MACHINERY 

and  is  intended  to  be  used  as  a  line  switch  as  well  as  for  starting 
the  motor.  The  lever  has  three  positions:  "off",  "starting",  and 
"running". 

To  start  the  motor,  the  switch  is  thrown  to  the  "starting  posi- 
tion", and  is  left  there  until  the  motor  reaches  nearly  full  speed  and 
then  the  swkch  is  quickly  thrown  over  to  the  "running  position". 
The  time  required  for  bringing  an  induction  motor  from  rest  up  to 
rated  speed  is  about  one  minute,  the  time  in  any  given  case  depend- 
ing upon  the  value  of  the  starting  voltage  used,  and  the  amount  of 
load  on  the  motor  at  starting.  In  the  "off  position"  both  compen- 
sator and  motor  windings  are  disconnected  from  the  line.  In  the 


Fig.  349. 


Diagram  of  Electrical  Connections  for  a  Two-Phase  Compensator  for  Starting 
Two-Phase  Induction  Motors 


"starting  position",  Fig.  348,  the  switch  connects  the  line  to  the 
terminals,  and  the  motor  to  the  taps,  of  the  compensator  winding, 
without  circuit  breakers  or  fuses  in  circuit.  In  the  "running  position" 
the  compensator  winding  is  cut  out  and  the  motor  is  connected  to  the 
line  through  suitable  fuses  or  overload  relays  mounted  directly  above 
the  compensator.  For  instance,  Fig.  347  shows  three  "cartridge" 
fuses,  one  in  each  of  the  three  line  wires.  To  prevent  the  attendant 
from  carelessly  throwing  the  motor  directly  on  the  line  at  starting, 
an  automatic  latch  is  provided,  so  arranged  that  the  lever  at  the 
"off  position"  can  be  thrown  only  into  the  "starting  position" 


ALTERNATING-CURRENT  MACHINERY 


389 


(backward),  and  can  be  thrown  into  the  "running  position"  (for- 
ward) only  by  a  quick  throw  of  the  lever. 

The  "no- voltage  release",  shown  in  Fig.  348,  is  an  electromagnet 
whose  laminated  plunger  holds  a  tripping  lever  which  engages  with 
the  lever  mounted  on  the  switch  shaft.  If  for  any  reason  the  supply 
voltage  is  cut  off  from  the  motor,  the  no- voltage  release  acts  promptly 
to  release  the  tripping  lever  which  in  turn,  through  the  action  of  a- 
strong  spring,  throws  the  operating  lever  to  its  "off  position",  where 
it  remains  until  the  supply  voltage 
is  again  restored,  and  the  motor  is 
to  be  started. 

Fig.  349  shows  a  diagram  of 
connections  for  a  two- phase  com- 
pensator for  starting  two-phase  in- 
duction motors.  In  this  case  the 
line  is  connected  to  the  ends  of 
each  coil,  and  the  starting  connec- 
tions of  the  motor  to  one  of  these 
ends  and  the  taps.  The  arrange- 
ment and  wiring  is  essentially  simi- 
lar to  the  three-phase  compensator. 

For  starting  constant  -  speed 
polyphase  motors  above  5  horse- 
power, a  starting  compensator  is 
generally  used,  but  below  5  horse- 
power it  is  customary  to  connect 
polyphase  motors  directly  to  the  Fig.  350.  Snyder  Drill  Press 

!  .  Driven  by  Induction  Motor 

supply  mains. 

The  direction  of  rotation  of  a  two-phase  motor  is  reversed  by 
reversing  the  connections  of  one  of  the  phases,  that  is,  by  reversing 
the  connections  of  the  two  wires  belonging  to  one-phase  supplying 
current  to  the  stator  windings  of  the  motor. 

The  direction  of  rotation  of  a  three-phase  (three-wire)  motor 
is  reversed  by  interchanging  the  connections  of  any  two  of  the 
three  wires  used  to  lead  the  three-phase  currents  to  the  stator 
windings  of  the  motor. 

Typical  Induction  Motor  Installations.  The  application  of 
induction  motors  to  the  driving  of  machine  tools  has  been  rapidly 


390  ALTERNATING-CURRENT  MACHINERY 

growing  in  favor.  Among  the  advantages  of  electric  driving  over 
mechanical  driving  through  shafting  and  belting  are:  increased  shop 
production,  economy  of  power,  ease  of  control,  better  and  more 
convenient  arrangement  of  machines,  and  saving  of  floor  space.  Fig. 
350  is  a  view  of  a  Snyder  21 -inch  drill  press  driven  by  a  constant- 
speed  two  horse-power  induction  motor.  Fig.  351  shows  a  24-inch 
Chandler  planer  driven  by  a  7|  horse-power  induction  motor  running 
at  1,200  revolutions  per  minute.  The  individual  motor  drive  as 


Fig.  351.     Chandler  Planer  Driven  by  Induction  Motor 

applied  to  planers  has  many  advantages,  among  them  being  the  use 
of  independent  motors,  one  for  planing  and  one  for  raising  or  lower- 
ing the  cross-head. 

A  typical  application  of  constant-speed  induction  motors  to  the 
driving  of  spindles  in  a  textile  mill  is  illustrated  in  Fig.  352.  A  large 
number  of  Crocker-Wheeler  5-h.p.  motors  are  directly  coupled  to 
the  shafts  which  drive  the  spindles  above,  thus  eliminating  all  over- 
head shafting,  hangers,  and  belts. 

An  application  of  variable-speed  induction  motors  having 
phase-wound  rotors  with  slip  rings  is  given  in  Fig.  353,  which  shows 
several  Westinghouse  500-h.p.  "type  HF"  induction  motors  operating 
Worthington  centrifugal  pumps  in  a  municipal  pumping  plant. 


ALTERNATING-CURRENT  MACHINERY  391 


Fig.  352.     Constant  Speed  Induction  Motors  Used  for  Driving  Spindles  in  a  Textile  Mill 


Fig.  353.      Westinghouse  500-H.  P.  Induction  Motors  Operating  Centrifugal  Pumps  in  a 
Municipal  Pumping  Plant 


392 


ALTERNATING-CURRENT  MACHINERY 


The  laminations  of  both  the  stator  and  the  rotor  of  large  induc- 
tion motors  are  provided  with  ventilating  ducts  through  which  air 
is  driven  by  centrifugal  action.  Fig.  353  shows  a  number  of  holes 
in  the  cast-iron  casing  through  which  the  air  flows  after  passing 
through  the  ventilating  ducts  between  the  laminations  of  the  rotor 
and  the  stator. 

Fig.  354  shows  an  induction  motor  driving  a  triplex  pump. 
This  induction  motor  has  a  starting  resistance  inside  of  the  rotor, 


Fig.  354.     Induction  Motor  Driving  Triplex  Pump 

and  the  switch  rod  and  knob  are  shown  projecting  from  the  rotor 
shaft  at  the  left. 

.  Fig.  355  illustrates  a  very  common  method  of  installing  in- 
duction motors  when  used  for  driving  line  shafting  in  shops  or 
factories.  The  motor  is  shown  bolted  in  an  inverted  position  to 
the  ceiling  by  means  of  lag  screws,  and  is  furnished  with  two  pulleys, 
each  belted  to  a  different  line  shaft. 

Since  induction  motors  require  no  adjustment  and  practically 
no  attention,  they  may  be  installed  where  direct-current  motors  are 


ALTERNATING-CURRENT  MACHINERY 


393 


not  suitable,  with  the  consequent  advantage  of  saving  valuable 
floor  space. 

Single=Phase  Induction  Motor.  When  an  induction  motor 
(two-phase  or  three-phase)  is  once  started,  and  is  running  at  full 
speed,  all  the  phases  but  one  may  be  disconnected  from  the  primary 
member  and  the  machine  will  continue  to  operate  and  to  carry 
approximately  70  per  cent  as  much  load  with  the  same  slip  aad 
temperature  rise  as  when  all  the  phases  are  connected  to  the  supply 
mains. 

An  induction  motor,  however,  will  not  start  when  one  phase 
only  of  its  primary  member  is  connected  to  single-phase  supply 


Fig.  355.     Induction  Motor  Suspended  from  the  Ceiling,  Driving 
Shafting  by  a  Belt 

mains.  Therefore,  when  it  is  to  be  operated  from  single-phase  mains, 
special  provision  for  starting  must  be  made.  An  induction  motor 
designed  to  operate  from  single-phase  mains  and  provided  with 
special  arrangements  for  starting,  is  called  a  single-phase  induction 
motor. 

Three  methods  of  starting  single-phase  induction  motors  are  in 
general  use,  as  follows:  (a)  by  hand;  (b)  by  phase  splitting;  (c)  by 
repulsive  action  of  field  on  armature  obtained  by  temporary  con- 
nections during  starting. 

(a)     Hand  Starting.     Very  small  induction  motors  may  be 


394  ALTERNATING-CURRENT  MACHINERY 

started  by  giving  a  vigorous  pull  on  the  belt  which  connects  the 
motor  to  the  machinery  which  it  drives. 

(b)  Split-Phase  Starting.  When  an  alternating  current  divides 
between  two  branches  of  a  circuit,  there  is  a  phase  difference  between 
the  currents  in  the  two  branches,  if  the  ratio  of  resistance  to  reactance 
is  different  in  the  two  branches.  This  is  especially  true  if  one  branch 
contains  a  condenser.  A  two-phase  motor  will  start  when  the  cur- 


Fig.  356.     Stator  of  Single-Phase  Induction  Motor — 
Holtzer-Cabot  Company 

rents  in  the  two  stator  circuits  are  less  than  90  degrees  apart  in  phase, 
although  the  starting  torque  grows  less  and  less  as  the  phase  differ- 
ence of  the  two  currents  decreases.  The  dephasing  of  the  two  parts 
of  a  single  alternating  current  in  two  dissimilar  branches  of  a  given 
circuit  is  called  phase-splitting,  and  a  single-phase  induction  motor 
may  be  arranged  to  start  as  a  two-phase  motor,  by  splitting  a  single- 
phase  current  and  using  the  two  parts  of  the  split  current  exactly 
as  one  would  use  two  genuine  two-phase  currents. 

Fig.  356  shows  the  stator  of  a  2-h.p.  single-phase  induction 
motor  of  the  Holtzer-Cabot  Electric  Company.     One  set  of  stator 


ALTERNATING-CURRENT  MACHINERY  395 

coils,  the  "working  coils",  consist  of  many  turns  of  coarse  wire 
occupying  three-fourths  of  all  the  stator  slots,  and  the  other  set  of 
stator  coils,  the  "starting  coils",  consist  of  fewer  turns  of  fine  wire 
occupying  one-fourth  of  all  the  slots. 

At  starting,  both  sets  of  coils  are  connected  to  the  single-phase 
supply  mains,  and  the  difference  in  the  resistance  and  the  reactance 
in  the  two  sets  of  coils  splits  the  single-phase  current  supplied,  suffi- 
ciently to  give  a  slight  starting  torque.  This  type  of  split-phase 
induction  motor  cannot  start  with  any  considerable  load,  hence 
the  load,  if  it  is  difficult  to  start,  should  be  thrown  on  to  the  motor 


Fig.  357.     Wagner  Single-Phase  Induction  Motor 

by  means  of  a  friction  clutch  after  the  motor  is  running  at  full  speed. 
The  rotor  used  in  the  Holtzer-Cabot  motor  is  of  the  squirrel-cage 
type. 

A  single-phase  motor  will  run  in  either  direction  equally  well, 
depending  only  upon  the  direction  in  which  it  is  started.  There- 
fore, the  hand-started  motor  may  be  started  in  either  direction. 
The  direction  of  starting  of  the  split-phase  motor  may  be  reversed 
by  reversing  the  connections  of  the  starting  winding. 

(c)  Repulsion  Motor  Starting.  If  an  ordinary  direct-current 
dynamo  were  provided  with  a  laminated  field  magnet,  and  if  its 
field  magnet  were  excited  by  an  alternating  current,  currents  would 


396 


ALTERNATING-CURRENT  MACHINERY 


be  induced  in  the  armature  windings  by  the  alternating  field,  pro- 
vided the  brushes  of  the  direct-current  machine  were  set  at  an 
angle  of  about  45°  (for  a  two-pole  machine)  from  their  proper  position 
for  collecting  a  direct  current.  These  currents  induced  in  the  arma- 
ture would  be  acted  upon  by  the  alternating  field  so  as  to  pro- 
duce a  torque  which  would  cause  the  armature  to  rotate.  A  self- 
starting,  single-phase,  alternating-current  motor  constructed  on 
this  principle  is  called  a  repulsion  motor.  It  is  not  entirely  satisfac- 
tory in  operation,  but  the  repulsion-motor  principle  furnishes  the 


Fig.  358.     Part  Section  of  Wagner  Single-Phase  Induction  Motor 

best  means  for  making  a  self -starting  single-phase  induction  motor, 
that  is,  a  motor  which  is  arranged  so  that  it  can  act  as  a  repulsion 
motor  while  starting,  and  which  by  changing  certain  inside  con- 
nections can  be  altered  into  an  induction  motor  when  it  reaches 
full  speed. 

Fig.  357  is  a  general  view  of  a  single-phase  induction  motor 
arranged  to  start  as  a  repulsion  motor,  and  built  by  the  Wagner 
Electric  Manufacturing  Company.  The  motor  shown  has  a  four- 
pole  stator  winding,  the  iron  stator  core  being  made  very  much  like 
the  core  of  an  ordinary  induction  motor,  namely,  in  the  form  of  a 
laminated  ring  closely  surrounding  the  armature,  and  slotted  on 
its  inner  face. 


ALTERNATING-CURRENT  MACHINERY  397 

The  armature  is  of  the  ordinary  direct-current  drum  type 
provided  with  a  disk  commutator  with  radial  commutator  bars. 
Four  (for  a  four-pole  machine)  short-circuited  brushes  are  pressed 
against  the  face  of  the  disk-shaped  commutator,  as  shown  in  Fig. 
357.  At  starting,  the  stator  winding  is  connected  to  the  single- 
phase  supply  mains,  and  the  machine  starts  as  a  repulsion  motor. 
Inside  of  the  armature  are  two  governor  weights  V,  Fig.  358,  which" 
are  thrown  outwards  by  the  centrifugal  force  when  the  machine 
reaches  full  speed,  thus  pushing  the  solid  copper  ring  K  into  con- 
tact with  the  inner  ends  of  the  commutator  bars  L,  and  thus  com- 
pletely short-circuiting  the  armature  winding.  At  the  same  time 
barrel  I,  which  is  pushed  endwise  by  the  governor  weights  and  which 
carries  the  short-circuiting  copper  ring  K,  pushes  the  brush  holder 
or  rocker  arm  endwise,  and  lifts  the  brushes  off  the  commutator. 

In  starting  the  Wagner  single-phase  motor,  the  supply  volt- 
age is  usually  reduced  to  a  fractional  part  of  the  full  running  voltage. 
This  is  accomplished  by  the  use  of  a  small  step-down  transformer, 
usually  an  autotransformer,  in  much  the  same  way  as  has  been  ex- 
plained in  connection  with  the  autostarter  or  starting  compensator 
for  two-phase  and  three-phase  induction  motors. 

Induction  Generator.  An  induction  motor  runs  as  a  motor 
at  a  speed  less  than  the  speed  of  the  rotating  magnetism  in  the 
stator  iron  (synchronous  speed).  When  the  motor  load  is  decreased, 
its  speed  approaches  synchronous  speed,  and  the  intake  of  power 
from  the  alternating-current  mains  falls  off  more  and  more.  If 
the  rotor  is  driven  by  an  external  source  of  mechanical  power,  it 
may  be  speeded  up  to  synchronism,  in  which  case  the  intake  of  power 
becomes  zero,  except  for  core  loss  in  the  stator  iron.  If  now  the 
rotor  is  speeded  above  synchronism  by  the  external  source  of  power, 
the  stator  windings  deliver  power  to  the  alternating-current  mains, 
provided  the  alternating-current  generator  remains  connected  to  the 
mains  to  fix  the  frequency.  When  an  induction  motor  is  so  used,  it  is 
called  an  induction  generator.  The  use  of  the  induction  motor  as  an 
induction  generator  is  not  of  much  commercial  importance. 

Frequency  Changer.  An  induction  motor  provided  with  a  rotor 
having  a  definite  winding  with  terminals  brought  out  to  collect  rings, 
see  Fig.  344,  may  be  used  as  a  so-called  frequency  changer.  When 
the  rotor  stands  still,  the  rotating  stator  magnetism  induces  electro- 


398  ALTERNATING-CURRENT  MACHINERY 

motive  forces  at  full  frequency,  that  is,  of  the  same  frequency  as 
the  alternating  currents  supplied  to  the  stator.  If  the  rotor  runs  at 
one-fourth  speed,  let  us  say,  the  relative  speed  of  the  rotor  and  the 
stator  magnetism  is  three-fourths  of  the  speed  of  the  latter,  and  hence 
electromotive  forces  of  three-fourths  full  frequency  are  induced  in 
the  rotor  windings.  If  the  rotor  is  run  backwards  at,  let  us  say, 
one-half  of  the  speed  of  the  stator  magnetism,  then  the  relative 
speed  of  the  rotor  and  the  stator  magnetism  is  one  and  one-half 
times  the  speed  of  the  stator  magnetism,  and  electromotive  forces 
of  one  and  one-half  times  full  frequency  are  induced  in  the  rotor 
windings. 

Example.  A  certain  induction  motor  runs  at  one-third  synchronous 
speed  (n'  =  $n,  page  377),  then,  ignoring  stator  losses,  all  of  the  power  de- 
livered to  the  stator  is  transmitted  to  the  rotor,  and  of  this  total  power  one- 
third  appears  as  mechanical  power  driving  the  rotor,  and  two-thirds  appears 
as  electrical  power  developed  in  the  rotor  windings.  This  electrical  power, 
ignoring  the  resistance  loss  in  the  rotor  windings,  is  delivered  to  the  rotor 
collecting  rings. 

Furthermore,  if  the  rotor  has  the  same  number  of  conductors  as  the 
stator,  then  the  electromotive  forces  between  collector  rings  are  two-thirds 
as  great  as  the  voltages  applied  to  the  stator  windings,  and  their  frequency  is 
two-thirds  as  great. 

If  the  rotor  of  an  induction  motor  is  driven  backwards  by  an  external 
source  of  power  at  one-half  synchronous  speed  (nf  =  —  $n),  then  all  of  the  elec- 
trical power  delivered  to  the  stator  together  with  the  mechanical  power  used 
for  driving  the  rotor,  appears  as  electrical  power  in  the  rotor  windings,  and 
the  rotor  voltages  are  one  and  one-half  times  as  great  in  value,  and  one  and 
one-half  times  as  great  in  frequency  as  the  voltages  applied  to  the  stator. 

The  stator  current  in  an  induction  motor,  or  a  frequency  changer,  is 
sufficient  at  no-load  to  magnetize  the  stator.  This  stator  current  is  called 
the  no-load  current  of  the  machine.  When  current  is  taken  from  the  rotor, 
an  equal  (and  opposite)  additional  current  is  taken  from  the  supply  mains 
by  the  stator  windings,  exactly  as  in  the  case  of  the  transformer. 

The  above  statements  are  based  on  the  assumption  that  the 
rotor  windings  are  exactly  like  the  stator  windings,  both  as  to  the 
number  of  conductors,  and  as  to  the  grouping  of  the  conductors 
into  separate  circuits  or  phases.  If  the  rotor  has  half  as  many 
conductors  as  the  stator,  the  rotor  voltages  are  halved  and  the 
rotor  currents  are  doubled,  other  things  being  equal. 

In  alternating-current  plants,  designed  primarily  for  the  trans- 
mission of  power,  and  hence  using  a  low  frequency  (e.  g.,  25  to  40 
cycles  per  second),  there  is  sometimes  a  need  for  a  limited  amount 


ALTERNATING-CURRENT  MACHINERY  399 

of  current  of  a  higher  frequency.  To  meet  such  conditions,  a  fre- 
quency suitable  for  lighting  purposes,  60  cycles  or  more,  may  be 
cheaply  and  easily  obtained  by  means  of  the  frequency-changer. 
This  is  essentially  an  induction  motor  as  explained  above,  the  rotor 
of  which  is  driven  mechanically  by  an  auxiliary  synchronous  motor 
in  a  direction,  usually  opposite  to  its  natural  rotation.  The  current 
of  lower  frequency  is  fed  to  the  stator  windings  and  the  curremTof 
higher  frequency  is  taken  out  of  the  rotor  windings  by  means  of 
collector  rings.  The  frequency  of  the  motor  current  will  depend 
on  the  speed  at  which  the  rotor  is  driven.  Thus,  if  the  rotor  is  driven 
at  its  rated  speed  but  in  a  direction  opposite  to  its  natural  rotation, 
the  frequency  of  the  current  delivered  by  it  to  the  collector  rings 
will  be  twice  the  normal,  or  if  run  at  half  the  normal  speed  in  its 
natural  direction,  the  frequency  will  be  one-half  the  normal.  To 
change  a  current  with  a  frequency  of  40  cycles  into  one  of  60,  the 
motor  would  be  run  at  one-half  speed  in  an  opposite  direction,  while 
to  obtain  60  cycles  from  a  25-cycle  current,  the  rotor  would  run 
nearly  one  and  one-half  times  the  rated  speed  in  an  opposite  direction. 

The  total  power  delivered  to  a  frequency  changer  is  partly 
electrical  power  delivered  directly  from  the  low  frequency  supply 
mains  to  the  stator  of  the  frequency  changer,  and  partly  mechan- 
ical power  delivered  by  belt  to  the  rotor  of  the  frequency  changer 
from  the  auxiliary  driving  motor.  The  power  output  of  the  fre- 
quency changer  is  wholly  electrical  and  in  the  form  of  increased- 
frequency  alternating  currents  from  the  rotor. 

The  electrical  power  delivered  to  the  stator  of  the  frequency 
changer  is 


and  the  mechanical  power  delivered  by  belt  to  the  rotor  of  the 
frequency  changer  is 


p*= 


where  P  is  the  total  power  delivered  to  the  machine,  being  a  little 
greater  than  the  total  power  delivered  by  the  machine  at  the  in- 
creased frequency ;  /  is  the  low  frequency  of  the  alternating  currents 
supplied  to  the  stator  of  the  machine ;  and  /'  is  the  higher  frequency 


400  ALTERNATING-CURRENT  MACHINERY 

of  the  alternating  currents  delivered  by  the  rotor  of  the  machine. 

Therefore,  the  rotor  of  the  frequency  changer  must  be  designed 
for  the  total  output  of  power  at  the  higher  frequency,  and  the  stator 
of  the  frequency  changer  must  be  designed  for  the  intake  of  the 
amount  of  power  Pe,  which  is  supplied  to  it  electrically. 

For  example,  a  frequency  changer  rated  at  100  kw.  to  change 
a  40-cycle  current  to  one  having  a  frequency  of  60  cycles  per  second 
would  be  made  up  as  follows:  An  auxiliary  driving  synchronous 
motor  rated  at  33.3  kw.  designed  to  take  current  from  40-cycle  mains 
at  a  speed  of,  say  600  r.  p.  m. ;  it  would,  therefore,  have  eight  poles. 
Its  armature  would  be  direct  connected  to  the  rotor  of  the  frequency- 
changer  which  would  be  rated  at  100  kw.  The  stator  of  the  frequency- 
changer  would  be  rated  at  66.7  kw.  and  would  be  supplied  with 
current  having  a  frequency  of  40  cycles  per  second.  If  wound  for 
four  poles,  the  synchronous  speed  (as  an  induction  motor)  of  the 
rotor  of  the  frequency-changer  would  be  normally  1,200  r.  p.  m. 
But  by  driving  the  rotor  (by  the  auxiliary  synchronous  motor)  at  a 
speed  of  600  r.  p.  m.  in  a  direction  opposite  to  its  natural  rotation, 
the  frequency  of  the  rotor  currents  would  be  that  due  to  an  equiva- 
lent speed  of  1,200+600=  1,800  r. p. m.;  corresponding  thus  to  a 
frequency  of  60  cycles  per  second. 

For  the  sake  of  a  simple  illustration,  the  ratings  as  given  above 
are  based  on  the  assumption  of  a  100  per  cent  efficiency,  which, 
of  course,  on  account  of  the  unavoidable  power  losses,  is  never 
realized  in  practice. 

It  is  evident  that  the  frequency-changer  can  at  the  same  time 
be  designed  to  change  the  electromotive  force  by  using  a  suitable 
number  of  turns  in  the  stator  and  rotor  windings.  It  can  also  be 
used  to  change  the  number  of  phases  of  the  system  by  providing  a 
rotor  wound  for  a  number  of  phases  different  from  that  of  the  stator. 
For  instance,  it  may  be  designed  to  convert  from  three-phase,  6,000 
volts,  and  25  cycles,  to  two-phase,  2,500  volts,  and  62.5  cycles.  On 
account  of  this  flexibility  the  frequency  changer  is  sometimes  called 
a  "general  alternating-current  transformer."  A  number  of  these 
frequency  changers  are  in  present  use,  but  on  account  of  excessive 
magnetic  leakage,  they  are  not  as  satisfactory  in  operation  as  motor- 
generators.  One  of  the  large  manufacturing  companies  is  now 
recommending  as  a  frequency  changer  a  motor-generator  consist- 


ALTERNATING-CURRENT  MACHINERY  401 

ing  of  a  polyphase  induction  motor  of  one  frequency  driving  mechan- 
ically an  alternator  designed  for  the  frequency  desired. 

COMPARISON  OP  SYNCHRONOUS  MOTOR  AND 
INDUCTION  MOTOR 

To  summarize  the  characteristic  behavior  in  service  of  synchro- 
nous and  induction  motors,  and  to  simplify  the  comparison  between 
them,  the  following  tabular  statement  in  parallel  columns,  prepared 
by  C.  F.  Scott,  is  given. 

The  induction  motor  chosen  for  comparison  with  the  synchronous 
motor  is  of  the  so-called  '  "squirrel-cage"  type,  started  by  applying 
a  low  electromotive  force  to  the  primary  winding.  The  description 
following  will,  of  course,  require  modification  in  some  particulars, 
if  the  secondary  is  furnished  with  adjustable  resistance,  but  these 
modifications  are  of  minor  importance  and  do  not  affect  the  general 
comparison. 

SYNCHRONOUS  MOTOR  INDUCTION  MOTOR 

Auxiliary  Apparatus  Required 

1.  A  starting  motor;  or,   if  self-  1.     A  two-way  main  switch  with 
starting,  some  form  of  resistance  or  autotransformers  giving  a  low  e.m.f. 
transformer    for    reducing    the    volt-  for  starting.     This   may  be   at   any 
age.  distance  from  the  motor. 

2.  An  exciter,  driven  by  the  mo-  2.     No  exciter  is  required, 
tor    or    otherwise,    with    circuits    to 

switchboard  and  motor. 

3.  Rheostats  for  exciter  and  mo-  3.     No  field  rheostats  are  required, 
tor. 

4.  Instruments  for  indicating  when  4.     No  instruments  are  required, 
field  current  is  properly  adjusted. 

5.  Main    switch   and   exciter  5.     No    exciter    switches    are    re- 
switches,  quired. 

6.  A   friction    clutch    is    required  6.     No  friction  clutch  is  required, 
in  many  cases.  as  the  motor  starts  its  load 

Construction 

1.  Armature  winding.  1.     Primary  winding. 

2.  Field  winding  with  many  turns.  2.     Secondary,  short-circuited. 
Liable  to   accident   from    "field   dis- 
charge" if  exciting  current  is  suddenly 

broken;  or  from  high  e.m.f.  by  induc- 
tion from  the  armature  if  the  field  cir- 
cuit is  open. 

3.  Collector  rings  and  brushes.  3.     No  moving  contacts  on  "squir- 

rel cage"  secondary. 


402 


ALTERNATING-CURRENT  MACHINERY 


Starting— Normal 


1.  Motor  is  brought  up  to  speed 
without   load;    if   starting   motor   is 
used,  the  main  motor  must  be  brought 
to  proper  speed  and  "synchronized"; 
if  self-starting,   the  starting   devices 
must  be  cut  out  of  circuit   at  the 
proper  time. 

2.  Exciter  is  made  ready  for  de- 
livering proper  current  and  the  motor 
field  must    be   excited,   adjustments 
being  made  by  rheostats  until  instru- 
ments give  proper  indication. 

3.  Load  is  thrown  on  by  friction 
clutch  or  other  means. 


1.     Throw  switch  to  starting  and 
then  to  running  position. 


2.  There  is  no  exciter.  (The  mo- 
tor is  magnetized  by  lagging  current 
from  the  generator.) 


3.     The  motor  starts  its  own  load. 


Starting — Abnormal 


1.  If    the    several    operations    in 
starting  be  performed  improperly  or 
in  wrong  order,  injury  may  result. 
If  a  starting  motor  is  used,  the  syn- 
chronizing may  be  attempted  at  an 
improper  speed  or  phase;  if  the  motor 
is  self-starting  and  it  is  connected  to 
the  circuit  without  the  starting  de- 
vices, a  large  current  will  flow  which 
may  induce  a  high  e.m.f.  in  the  field 
circuit;  if  the  field  circuit  be  open,  a 
high  e.m.f.  may  be  induced  in  it  at 
other  times  also. 

2.  If  a  load  having  inertia  be  ap- 
plied by  closing  the  friction  clutch 
too  quickly  the  motor  may  be  over- 
loaded and  stopped. 

3.  If  motor  stops  owing  to  failure 
of  current  supply,  it  is  not  self-start- 
ing when   the   current  returns.     An 
attendant    is    always    required    for 
starting. 


1.  The  only  possible  error  is  in 
starting  with  the  switch  in  the  run- 
ning or  full  voltage  position,  which 
simply  causes  the  motor  to  exert  a 
greater  torque  and  consume  a  greater 
current  than  is  necessary. 


2.     The  motor  starts  its  own  load 
and  requires  no  friction  clutch. 


3.  The  motor  will  stop  if  the  cur- 
rent is  cut  off  at  the  power  house 
and  then  start  again  when  the  cur- 
rent is  supplied  to  the  circuit. 


Starting  and  Maximum  Running  Torque 


1.  The  starting  torque  of  the  self- 
starting  motor  is  very  small  and  an 
excessive  current  is  required  for  de- 
veloping it.  The  motor  starts  as  an 
induction  motor,  but  inefficiently,  as 
the  design  which  is  best  for  synchro- 
nous running  is  not  good  for  starting. 


1.  The  starting  torque  is  adjusta- 
ble and  may  be  several  times  full  load 
torque. 


ALTERNATING-CURRENT  MACHINERY 


403 


2.  The  maximum  torque  is  sev- 
eral times  the  full  load  torque,  and 
occurs  at  synchronous  speed;  below 
this  speed  the  torque  is  very  small; 
any  condition  which  momentarily 
lowers  the  speed  causes  the  motor  to 
stop. 

Speed 


2.  The  maximum  torque  is  usu- 
ally greater  than  that  of  the  syn- 
chronous motor,  but  it  occurs  at  a 
reduced  speed  and  there  is  a  large 
torque  at  lower  speeds. 


1.  The  motor  has  a  single  definite 
speed;  at  other  speeds  its  torque  is 
very  small,  and  the  current  is  very 
large. 


1.  The  motor  may  be  designed  for 
a  practically  constant  speed,  with 
large  torque  at  lower  speeds;  or  for 
several  definite  speeds  by  changing 
the  number  of  poles;  or  for  variable 
speed,  for  cranes,  elevators,  hoists, 
and  the  like. 
Current 


1 .  If  there  is  useful  starting  torque, 
the   current   required   for   producing 
it  is  very  great. 

2.  The   running  current  depends 
upon   the  wave  form.     If  the  wave 
form  of  the  motor  and  of  the  circuit 
differ,  a   corrective   current  will  fol- 
low, which  cannot  be  eliminated  by 
adjustment  of  field  excitation. 

3.  The  running  current  depends 
upon  uniformity  of  alternations  of  the 
current,  i.e.,  upon  the  uniformity  of 
the  speed  of  the  generator  and  other 
synchronous  motors.     The  motors  at- 
tempt to  follow  the  generator  speed 
exactly.     If  the  latter  pulsates,  the 
motors    pulsate    also;    they    vibrate 
about  a  mean  position,  "hunting"  or 
"pumping."     One     motor     pumping 
incites    others.     The    current    is    in- 
creased even  though    the   conditions 
may  still  be  operative. 

4.  The  running  current  depends 
upon  the  relation  between  the  field 
current  (which  is  adjusted  by  the  at- 
tendant) and  the  e.m.f.  of  the  circuit. 
The  main  current  may  be  made  lead- 
ing or  lagging  or  theoretically  it  may 
be  neither.    The  e.m.f.  of  the  circuit  is 
an  element  which  is  under  the  partial 
control  of  the  attendants  at  every  mo- 
tor, as  well  as  at  the  generator  station. 


1.  The  starting  current  may  be 
made  proportional  to  the  torque,  and 
is  1  \  to  2 1  times  that  required  for  the 
same  torque  at  high  speed. 

2.  The   running   current  is  prac- 
tically independent  of  the  difference 
in   wave   form,    as   it   has   no  wave 
form  of  its  own. 


3.  The  current  is  practically  in- 
dependent of  fluctuations  in  gener- 
ator speed,  as  there  is  a  slip  between 
the  synchronous  and  the  actual  speed 
of  the  motor. 


4.  The  current  is  not  subject  to 
any  adjustments  which  the  motor  at- 
tendant can  make,  nor  is  the  e.m.f. 
of  the  circuit  in  any  way  under  his 
control. 


404 


ALTERNATING-CURRENT  MACHINERY 


Power  Factor 


1.  As  the  power  factor  is  the  re- 
lation   between    actual    current    and 
energy  current,  it  is  dependent  upon 
wave  form,  hunting,   and  field   cur- 
rent.    Under    favorable    conditions, 
the  motor  may  have  a  high  power 
factor;  under  many  actual  conditions 
it  may  not;  under  some  conditions 
the  highest  attainable  power  factor  is 
less  than  that  of  the  induction  motor. 

2.  The  current  may  be  lagging  or 
leading,   depending  upon  the  motor 
field  strength. 


1.  The  power  factor  varies  with 
load,  but  is  definite  and  is  practi- 
cally independent  of  wave  form  and 
hunting. 


2.     The   current   to   the   motor  is 
always  a  lagging  current. 


Reaction  Upon  Generator  and  Circuit 


1.     The  motor  impresses  its  own 
wave  form  on  the  circuit. 


2.  A  motor  may  augment  the  fluc- 
tuations in  generator  speed  by  the  os- 
cillation of  its  own  armature.     One 
motor  may  increase  the  disturbance 
in  the  circuit  so  as  to  interfere  with 
other  motors  not  otherwise  seriously 
affected. 

3.  As  the  current  may  be  either 
lagging  or  leading,  the  drop  in  e.m.f. 
in  the  generator,  and  between  gener- 
ator and  motor  may  be  either  more 
or  less  than  that  which  could  be  caused 
by  a  non-inductive  load  or  by  an  in- 
duction motor. 

4.  If  a  short-circuit  occurs  in  the 
transmission  system,  the  motor  acts 
as  a  generator,  which  thereby  greatly 
increases  the  current  and  the  intensity 
of  the  short-circuit. 

5.  If  the  circuit  is  opened,  either 
by  a  switch,  a  circuit  breaker,  a  fuse, 
or  the  breaking  of  the  line,  the  mo- 
tor speed  falls,  its  e.m.f.  is  no  longer 
in  phase  with  that  of  the  circuit;  the 
two  are  thereby  added,  thus  doubling 
the  normal  e.m.f.   and  bringing  in- 
creased strains  on  the  insulation  and 
the  opening  devices. 


1.  The  motor  has  no  wave  form 
to  impress  upon  the  circuit;  its  tend- 
ency is  to  smooth  out  irregularities 
in  a  wave  not  a  sine. 

2.  The  motor  has  a  damping  ac- 
tion upon  fluctuations  in  frequency; 
in  some  cases  a  synchronous  motor 
which  hunts  may  run  smoothly  when 
an  induction  motor  is  connected  to 
the  same  circuit. 

3.  The  drop  in   e.m.f.   is  always 
greater  than  would  be  caused  by  non- 
inductive  load. 


4.     The  motor  does  not  generate 
current  when  there  is  a  short-circuit. 


5.  The  motor  does  not  generate 
e.m.f.  when  it  is  disconnected  from 
the  circuit. 


ALTERNATING-CURRENT  MACHINERY 


405 


Causes  which  May  Accidentally  Stop  a  Motor 


1.  Momentary  lowering  of  e.m.f. 
caused  by  short-circuit  on  the  line, 
or  by  accident  to  another  motor,  or 
by  error  in  synchronizing  a  generator, 
or  by  the  "switching  over"  of  the  mo- 
tor from  one  circuit  to  another,  is  apt 
to   cause  the  motor,   particularly  if 
carrying  load,  to  fall  from  synchron- 
ism and  stop. 

2.  A  heavy  load,  even  momentary, 
may  exceed  the  limiting  torque  and 
cause  the  motor  to  drop  from  syn- 
chronism, even  though  the  load  be 
removed  immediately.     The  connec- 
tion between  generator  and  motor  is 
rigid. 

3.  If  the  generator  speed  suddenly 
increases,    a   motor   carrying   a  load 
having  inertia  may  be  unable  to  in- 
crease its  speed  quickly  without  ex- 
ceeding the  limiting  torque,  which  will 
cause  the  motor  to  stop. 

Summary 


1.     Momentary  lowering  of  e.m.f. 
causes  momentary  decrease  in  speed. 


2.  An  excessive  load  receives  the 
stored  energy  of  the  motor  and  of  the 
load  itself  as  the  motor  speed  falls; 
when    the    excess    load    is    removed 
the  motor  speed  increases  again.     The 
connection    between    generator    and 
motor  is  elastic. 

3.  The     motor     readily     follows 
changes  in  generator  speed. 


1.  The  motor  is  an  active  element 
in  the  system;  it  acts  as  a  generator 
in  impressing  its  own  wave  form,  its 
e.m.f.  and  its  fluctuations  upon  the 
circuit.     These  fluctuations  may  be 
caused  by  an  intermittent  load. 

2.  The    m^tor   is  a  sensitive  ele- 
ment   in    the  system.    Its  successful 
operation  is  dependent  upon  a  proper 
relation  between  the  design  of  the  mo- 
tor itself  and  of  other  machines  in  the 
system.     Its  successful  operation  also 
depends  upon  the  proper  adjustment 
and  freedom  from  speed  fluctuation 
in  generators  and  other  motors.     It  is 
liable  to  momentary  variations  from 
normal  conditions,  such  as  a  sudden 
over-load  and  sudden  increase  of  gen- 
erator speed  or  a  momentary  fall  in 
e.m.f. 

3.  The  motor  requires  skill  and 
care  on  the  part  of  the  attendant  for 
starting,  for  readjusting  and  for  keep- 
ing the  various  brushes  and  auxiliary 
apparatus  in  condition. 


1.  The  motor  is  a  passive  element 
in  the  system.  Each  motor  attends 
to  its  own  work  and  does  not  try  to 
run  the  system. 


2.  The  motor  is  not  sensitive  to 
differences  in  the  design  of  other  ap- 
paratus operating  on  the  same  system. 


3.  No  experience  and  electrical 
skill  are  required  of  the  attendant  and 
there  is  little  or  nothing  to  get  out  of 
order  either  through  carelessness  or 


406  ALTERNATING-CURRENT  MACHINERY 

4.  The  power  factor  is  under  the  4.     The  motor  has  a  definite  power 
control  of  the  operator  and  the  cur-  factor,  depending  upon  the  load;  the 
rent  may  be  made  leading  or  lagging.  out-of-phase   current  does  not   vary 
Instruments  are  necessary  in   order  greatly  at  different  loads.     Thechang- 
that  proper  adjustments  may  be  made  ing  load,  therefore,  has  comparatively 
by  the  attendant.  little  effect  upon  the  drop  in  voltage 

and  in  regular  service  there  is  little 
liability  that  the  motor  will  disturb 
the  e.m.f.  of  the  circuit. 

5.  The  motor  and  its  operation          5.     The  motor   and  its  operation 
are  complex  and  involve  many  pos-      are  simple  and  reliable. 

sibilities  of  accident. 

The  synchronous  motor  is  obviously  not  suitable  for  general 
distribution  of  power,  owing  particularly  to  its  lack  of  starting 
torque,  the  skill  required  in  attendance,  and  the  liability  of  the  motor 
to  stop  if  the  conditions  become  abnormal.  These  objectionable 
features,  however,  are  of  much  less  importance  when  motors  are 
installed  in  substations  or  are  of  sufficiently  large  size  to  justify  an 
attendant. 

The  characteristic  of  the  synchronous  motor  which  may  be 
particularly  advantageous  is  the  fact  that  the  power  factor  of  the 
current  can  be  varied  and  that  the  current  may  be  made  leading. 

INDUCTION  MOTOR  TESTS 

Heat  Test.  The  heat  test  on  induction  motors  is  usually  carried 
out  by  connecting  the  terminals  of  the  stator  windings  to  mains 
of  the  proper  voltage  and  frequency,  and  taking  rated  full-load 
mechanical  power  from  the  rotor  until  a  constant  temperature  has 
been  reached.  Then  the  motor  is  shut  down,  and  the  temperatures 
of  the  following  parts  are  taken: 

Armature  laminations 

Armature  conductors 

Field  conductors 

Field  laminations 

Frame 

Bearings 

Room 

•  If  the  motor  is  small,  the  mechanical  power  output  may  be 
absorbed  by  a  brake.  For  large  power  motors,  however,  a  brake 
becomes  troublesome.  In  any  case,  the  most  convenient  way  to 


ALTERNATING-CURRENT  MACHINERY  407 

measure  the  power  is  to  belt  to  the  motor  a  direct-current  sepa- 
rately-excited generator,  whose  losses  can  be  easily  determined,  and 
measure  the  ouput  of  this  generator.  The  output  of  the  motor  is 
equal  to  the  output  of  the  generator  plus  the  losses  in  the  gener- 
ator. The  losses  in  the  generator  are:  the  I2R  loss  in  the  armature, 
brush  and  bearing  friction,  and  windage  loss,  and  the  core  loss  due 
to  hysteresis  and  eddy  currents. 

The  field  being  separately  excited,  the  field  loss  need  not  be 
considered.  The  field  current  must,  however,  be  kept  constant. 

During  the  heat  run,  the  following  observations  are  regularly 
recorded : 


MOTOR 

GENERATOR 

Volts 

Amperes 

Speed 

Volts 

Amperes 
Armature 

Amperes 
Field 

Speed 

The  PR  loss  in  the  generator  armature  can  be  calculated  from 
the  current  flowing,  and  the  resistance  of  the  armature. 

To  determine  the  stray-power  losses  (equal  to  all  the  losses 
except  PR)  proceed  as  follows: 

Disconnect  the  motor  from  the  alternating-current  mains,  and  run  the 
combination  of  motor  and  generator  from  the  direct-current  end,  at  the  same 
speed  as  that  recorded  during  the  run,  the  direct-current  machine  being  now 
used  as  a  motor.  Its  field  is  excited  to  the  same  value  that  it  had  during 
the  run.  The  speed  is  adjusted  to  the  right  value  by  varying  the  voltage 
supplied  to  the  direct-current  machine  used  as  a  motor.  When  the  speed 
is  correct,  record  the  amperes  and  volts  taken  by  the  direct-current  machine. 
Next  the  belt  is  thrown  off,  and  the  voltage  supplied  to  the  armature  termi- 
nals of  the  direct-current  motor  is  adjusted  until  proper  speed  is  attained, 
the  field  current  remaining  as  before,  and  the  input  of  power  is  again  recorded. 

Let  Wi  =  the  watts  input  to  the  direct-current  motor  with  the  belt  off; 
Wz  —  watts  at  same  speed  to  drive  both  machines  with  belt  on;  C  =  core  loss 
of  direct-current  machine;  F  =  friction  of  direct-current  machine  without  belt; 
FI=  friction  of  induction  motor  without  belt;  /  =  increase  of  bearing  friction 
of  direct-current  machine  due  to  belt  tension;  and  /i=  increase  of  friction  of 
bearings  of  induction  motor  due  to  belt  tension.  Then 

Wi  =  C  +  F  (t) 

and 

W2=C  +  F  +  F1+f1+  (ii) 


Subtracting  (i)  from  (ii)  we  get 


or 


408  ALTERNATING-CURRENT  MACHINERY 

Now,  since  the  two  machines  are  of  about  the  same  size,  we  may  assume 
that  the  increase  in  friction  of  each,  due  to  belt  tension,  is  the  same,  so  that 

TF2  -  Wi  -  F! 


But  Fi,  the  friction  of  the  induction  motor,  can  be  determined  as  de- 
scribed under  core-loss  test.  Hence,  we  have  the  following  expression  for 
the  stray-power  loss  of  the  generator: 

TF,  -TF2-Fi 
stray-power  loss  =  W  \  -\  —      —  -  — 

total  output  of  motor  =  stray-power  loss  +  El  +  I2R 

This  expression  gives  an  exact  method  of  determining  the  output  of  any  kind 
of  motor,  by  using  a  direct-current  generator  as  a  load.  If  Fi  is  unknown, 
we  may  neglect  the  increase  of  bearing  friction  due  to  belt  tension,  giving 
results  sufficiently  accurate  for  a  heat  run. 

Breakdown  Test.  The  breakdown  test,  as  in  the  case  of  a  syn- 
chronous motor,  is  to  determine  the  maximum  output  of  the  motor, 
that  is,  the  load  which  will  cause  the  motor  to  "break  down",  and 
stop.  To  make  this  test  the  load  on  the  motor  is  increased,  until 
the  motor  breaks  down,  the  maximum  output  being  noted.  It  is 
essential  that  the  alternating  currents  be  supplied  to  the  motor  at 
normal  voltage.  As  the  torque  and,  therefore,  the  maximum  output, 
varies  as  the  square  of  the  voltage,  the  results  obtained  by  this  test 
will  not  be  accurate  unless  the  voltages  applied  to  the  stator  windings 
are  kept  constantly  at  normal  value.  If  for  any  reason  it  is  impos- 
sible to  load  the  motor  to  the  breakdown  point,  the  voltage  may  be 
reduced,  and  the  maximum  load  at  normal  voltage  may  be  calcu- 
lated from  the  value  obtained  at  the  reduced  voltage  by  multiplying 

.      .  .     /  normal   voltage  \2 

the  observed  maximum  load  by  I  -  I  . 

V  reduced  voltage  / 

If,  for  example,  an  induction  motor  rated  at  50  horse-power 
gives  a  maximum  output  of  25  horse-power  at  one-half  its  rated 
voltage,  its  maximum  output  at  normal  voltage  would  be  approxi- 
mately 100  horse-power. 

The  most  convenient  way  to  load  the  motor  in  the  above  test 
is  to  belt  it  to  a  direct-current  generator,  as  previously  explained. 
To  determine  the  output  of  the  induction  motor,  from  observations 
on  the  direct-current  generator,  proceed  in  the  same  manner  as 
described  under  heat  run. 


ALTERNATING-CURRENT  MACHINERY 


409 


In  commercial  work  it  is  not  customary  to  run  this  test  to  the 
breakdown  point.  The  usual  method  is  to  find  out  if  the  motor 
will  stand  50  per  cent  overload  without  breaking  down.  When,  how- 
ever, it  is  desired  to  obtain  a  full  set  of  data  on  a  machine,  the  test 
is  carried  to  the  breakdown  point. 

Starting  Torque  Test.  The  stationary  or  starting  torque  de- 
veloped by  a  motor  determines  the  amount  of  load  under  which 
it  will  start.  To  perform  this  test  a  brake  is  clamped  to  the  pulley 
as  shown  in  Fig.  359.  A  is  the  center  of  the  pulley;  B  is  the  point 
of  suspension  of  the  brake  arm  from  the  spring  dynamometer  S; 
C  is  a  reference  pointer,  carried  on  a  standard,  on  a  horizontal  line 
through  the  center  of  the  pulley.  The  spring  S  will  measure  the 


Fig.  359.     Prony  Brake  Arrangement  for  Testing  Starting  Torque 

tangential  force  exerted  by  the  motor  at  a  radius  AB  when  B  has 
moved  down  to  the  point  C,  and  the  spring  dynamometer  S  is  sus- 
pended vertically. 

The  reading  of  S  will  be  affected  by  the  friction  in  the  motor 
bearings.  The  following  procedure  will  eliminate  this  error:  Fasten 
to  the  brake  arm  a  weight  W,  sufficient  to  overcome  the  friction 
of  the  bearings,  so  that  the  arm,  if  left  unsupported,  will  always 
be  carried  downward  by  its  weight.  By  slowly  raising  and  lowering 
the  brake  arm  by  means  of  the  cord  which  supports  the  spring  dyna- 
mometer S  (no  current  passing  through  the  motor),  we  get  two 
different  readings  of  S  when  B  passes  the  point  C.  While  the  arm 
is  being  raised  the  friction  in  the  bearings  acts  in  the  same  direction 


410  ALTERNATING-CURRENT  MACHINERY 

as  the  weight  W.  Second,  as  B  passes  (7,  while  the  arm  is  being 
lowered,  the  friction  of  the  bearing  acts  against  the  direction  of  the 
pull  due  to  W. 

Let  W—  pull,  exerted  by  the  weight  of  the  brake  and  arm  weight; 
F=  friction  of  the  bearings;  a  =  scale  reading  when  arm  is  being 
raised;  and  6=  scale  reading  when  arm  is  being  lowered.  Then 

W+F=a 
and 

W-F=b 


Let  T  be  the  force  exerted  by  allowing  the  current  to  act  on 
the  motor,  this  force  being  in  the  same  direction  as  W.  Then, 
with  current  flowing  in  the  motor,  let  c=  scale  reading  while  arm  is 
being  raised;  and  d=  scale  reading  while  arm  is  being  lowered.  Then 

T+W+F=c 
and 

T+W-F=d 
or 


But  from  above  we  have 

w-a+b 

2 

substituting  this  value  of  W,  we  have 

T  =  C~^~d       a 


By  this  method  both  the  weight  of  the  brake  arm  and  the  friction 
of  rest  in  the  bearings  are  eliminated. 

To  carry  out  this  test,  this  set  of  observations  is  repeated  for 
as  many  values  of  the  current  in  the  motor  as  desired.  The  current 
should  range  at  least  from  one-half  to  twice  the  normal  full-load 
current.  To  obtain  the  desired  current,  the  voltage  across  the  motor 
terminals  must  be  adjusted. 

Core  Loss  Test.    The  core  loss  of  an  induction  motor  cannot 


ALTERNATING-CURRENT  MACHINERY 


411 


be  measured  by  the  method  used  in  the  case  of  a  synchronous  motor 
or  alternating-current  generator,  namely,  by  driving  it  by  a  direct- 
current  motor  and  observing  the  motor  input  for  various  voltages 
generated,  since  an  induction  machine  will  not  generate  electro- 
motive force  unless  it  is  connected  to  the  mains.  If  it  is  connected 
to  the  mains,  the  mains  may  supply  power  to  it,  and  hence  the 
power  supplied  by  the  direct-current  motor  is  not  the  total  power- 
delivered  to  the  machine. 

The  core  loss  is  measured  in  the  same  manner  as  in  the  case 
of  a  transformer,  namely,  by  connecting  it  to  the  alternating-current 


200 

Q 
/ 

160 

/ 

/ 

'o 

/ 

120 
100 
80 

I 

/ 

& 

/ 

/ 

* 

c 

X 

/ 

60 
40 
20 

0<x 

/ 

^ 

^ 

>•  

—  —  — 

—     •- 

,_-  — 

*~*~~~~ 

O  100  200  300  400  500  600  700 

l/o/fs 

Fig.  360.     Curve  Showing  Watts  Input  to  a  1-H.  P.  Motor  at  Different  Voltages 

mains  at  normal  voltage  and  frequency,  and  measuring  the  watts 
input  at  no-load.  There  is  this  difference,  however,  between  the 
case  of  the  transformer  and  that  of  an  induction  motor,  that,  whereas 
in  the  former  the  power  input  is  practically  all  needed  to  supply  the 
core  loss,  in  the  latter,  a  part  of  the  power  input  goes  to  supply  the 
friction  and  windage  losses  of  the  motor,  and  the  appreciable  PR 
loss  in  the  stator  windings. 

When  an  induction  motor  is  run  at  no-load,  the  speed  remains 
practically  constant  as  the  voltage  is  reduced,  until  the  motor  "breaks 
down"  and  stops.  The  power  input  to  the  motor  (at  no-load)  at  any 
voltage  consists  of  the  core  loss  at  that  voltage  plus  the  friction 


412  ALTERNATING-CURRENT  MACHINERY 

and  windage  loss  plus  the  I2R  losses  in  the  stator  and  rotor  wind- 
ings. But  the  friction  and  windage  loss  is  nearly  constant  at  con- 
stant speed,  therefore  the  watts  input  to  the  motor  at  various  voltages 
consists  in  part  of  the  constant  friction  and  windage  loss,  and  in  part 
of  the  variable  core  loss.  Such  a  series  of  observations  on  a  1-h.  p., 
550-volt,  3-phase  motor  is  shown  plotted  in  Fig.  360.  The  voltage 
can  be  reduced  to  the  point  at  which  the  motor  breaks  down;  beyond 
this  we  cannot  go.  As  the  ordinates  on  this  curve  are  equal  to  the 
sum  of  a  constant  and  a  variable  part,  and  since  at  zero  voltage  the 
variable  part  becomes  zero,  it  follows  that  if  we  prolong  the  curve  as 
shown  in  the  dotted  portion  until  it  crosses  the  axis  of  watts,  that  is 
for  zero  volts,  the  value  intercepted  on  the  axis  of  watts  may  be  con- 
sidered, without  much  error,  to  be  the  constant  part  of  the  watts, 
namely,  the  friction  and  windage  loss.  Thus  in  Fig.  360,  26  watts 
represents  the  power  lost  due  to  friction  and  windage  in  the  case  of 
the  induction  motor  above  mentioned.  The  sum  of  the  core  loss, 
friction  losses,  and  PR  loss  at  normal  voltage,  viz,  550  volts,  is 
found  from  the  curve  to  be  109  watts.  For  each  observed  value  of 
the  volts  and  the  watts,  the  current  per  phase  to  the  stator  wind- 
ings must  be  recorded.  Then,  the  resistance  of  the  stator  wind- 
ings having  been  measured,  the  PR  loss  corresponding  to  any  given 
voltage  can  be  calculated.  To  obtain  the  core  loss  corresponding 
to  any  voltage,  we  must,  therefore,  subtract  the  constant  friction 
losses  plus  the  PR  loss  in  the  stator  windings  from  the  total  ob- 
served input  to  the  motor  (when  running  unloaded). 

For  example,  in  the  case  of  the  1-h.  p.  induction  motor  under 
consideration,  the  current  input  per  phase  at  no-load  was  measured 
and  found  to  be  0.655  amperes,  when  the  voltage  between  supply 
mains  was  550  volts.  The  resistance  per  phase  was  also  measured 
and  found  to  be  15.5  ohms.  The  total  PR  loss  at  no-load  was, 

therefore,  3  X  O65?  X  15.5  =  20  watts. 
Therefore,  at  normal  voltage 

core  loss  =  109  -  26  -  20  =  63  watts 

Impedance  Test.  This  test  is  carried  out  in  the  same  manner 
as  the  core-loss  test.  The  motor  is  connected  to  the  alternating- 
current  supply  mains  and  the  amperes  flowing,  the  watts  input, 
and  the  volts  at  the  terminals  of  the  motor  are  measured,  the  watts, 


ALTERNATING-CURRENT  MACHINERY 


413 


of  course,  being  measured  by  wattmeters.  In  this  test,  however, 
the  motor  is  not  allowed  to  run  free,  but  its  armature  is  blocked 
to  prevent  it  from  turning.  Instead  of  supplying  normal  voltage 
to  the  motor,  the  voltage  supplied  is  cut  down  to  only  five  or  ten 
per  cent  of  the  normal  value  and  is  then  raised  carefully  until  the 
ammeters  show  about  one-third  to  one-half  of  the  full-load  current. 
The  motor  must  be  supplied  with  current  at  normal  frequency.  The 
following  observations  should  be  recorded: 

Amperes  in  each  line 
Volts  for  each  phase 
Total  watts 

The    observations    are    repeated    with    increasing    value    of    the 

voltages,  until  the  current  has  reached  from  one  and  one-half  to 

over  twice  the   full-load  value. 

Fig.  361  shows  curves  of  observa- 

tions taken  in  this  way  for  the 

1-h.  p.,  550-volt,  three-phase  mo- 

tor referred  to  above.  This  motor 

takes  one  ampere  of  current  per 

phase  at  full-load  with   normal 

voltage  of  550  volts.     The  two 

curves  are  plotted  with  current 

and  volts,  and  with  watts  and 

volts  as  ordinates  and  abscissas, 

respectively.    The  former  curve 

is  the  straight  line. 

Since   for    full-load  current 
with  the  armature  standing  still, 


between  Terminate 


Fig.  361. 


Curve  for  Impedance  Test  of 
Induction  Motor 


the  voltage  applied  to  the  termi- 
nals of  the  motor  is  very  low,  the 
number  of  watts  supplied  to  overcome  core  loss  is  very  low.  In  fact, 
practically  all  the  watts  supplied  are  used  up  in  heating  the  con- 
ductors of  the  stator  and  rotor.  The  watts  input,  therefore,  for 
normal  rated  full-load  current  but  with  rotor  blocked,  may  be  taken 
as  a  measure  of  the  total  PR  losses  (primary  and  secondary)  of  the 
entire  machine  at  full-load.  For  the  motor  for  which  the  curves 
are  shown,  the  PR  losses  at  full-load  current  are  equal  to  100  watts. 
Efficiency  Test.  As  in  the  case  of  the  machines  previously 


414          ALTERNATING-CURRENT  MACHINERY 

considered,  the  efficiency  of  an  induction  motor,  at  a  given  load  is 
equal  to  the  output,  divided  by  the  output  plus  the  losses,  at  that 
load.  All  the  losses  can  be  determined  from  the  core-loss  test  and 
the  impedance  test;  the  core  loss  and  friction  and  windage  losses 
being  determined  from  the  former,  and  the  copper  losses  from  the 
latter.  For  the  three-phase,  1-h.  p.  motor  above  mentioned,  the 
losses  at  full  load  are  as  follows  : 

Friction  and  windage  loss  =     26  watts 

Core  loss  =     63  watts 

Copper  loss  (72/2)  =  100  watts 

Total  losses  =  189  watts 

therefore, 

746 
efficiency  at  1-h.  p.  output  =  =  79.8% 

/rrO  ~| 


The  efficiency  of  large  machines  is  generally  calculated  in  this 
way.  For  smaller  machines  it  is  more  usual  to  determine  the  effi- 
ciency by  actually  measuring  input  and  output.  The  reason  for 
this  is  that  the  actual  losses  occurring  in  the  motor  when  it  is  loaded, 
are  different  from  the  values  as  calculated  from  results  of  tests  at 
no-load.  The  differences  between  these  calculated  and  actual  losses 
are  comparatively  large  in  a  small  machine. 

To  test  the  efficiency  by  measuring  the  total  input  and  out- 
put, the  most  convenient  method  is  to  belt  the  motor  to  a  direct- 
current  generator,  measuring  the  output  of  the  generator,  and  the 
input  to  the  motor.  The  output  of  the  motor  is,  of  course,  equal  to 
the  output  of  the  generator  plus  the  generator  losses.  The  losses  of 
the  generator  may  be  calculated  as  described  under  the  heat  test. 

The  output  and  input  of  the  motor  are  measured  for  various 
loads  successively  from  a  very  small  load  to  perhaps  50  per  cent 
overload,  so  that  a  curve  may  be  constructed  showing  the  relations 
between  efficiency  and  load.  The  losses  of  the  generator  must  be 
determined  at  each  speed  occurring  in  the  series  of  observations, 
because  the  generator  losses  vary  with  the  speed. 

Slip  Test.    The  slip  at  a  given  load  on  the  motor  is  the  ratio 


where  n  is  the  synchronous  speed  of  the  motor,  and  nf  is  the  speed 


ALTERNATING-CURRENT  MACHINERY  415 

under  the  given  load.  Slip  is  usually  expressed  in  per  cent,  in  which 
case 

per  cent  slip  =  -       -  X  100 
n 

The  synchronous  speed  in  revolutions  per  second  is  equal  to 

2f 

— — ,  where  /  is  the  frequency  of  the  alternating  currents  supplied 
P 

to  the  stator,  and  p  is  the  number  of  "poles"  of  stator  magnetism, 
NS,  NS  in  Figs.  333,  334,  and  335.  The  slip  is  independent  of 
the  number  of  phases. 

The  speed  of  the  motor  at  zero-load  is  very  nearly  equal  to  the 
synchronous  speed,  and  where  /  and  p  are  not  known,  the  zero-load 
speed  may  be  used  for  n  without  great  error. 

The  determination  of  the  slip  of  an  induction  motor  at  full- 
load  with  full-rated  voltage  applied  to  the  stator  windings,  is  an 
important  test.  This  slip  may  be  determined  by  observing,  by 
means  of  a  speed  counter  or  tachometer,  the  speed  n'  of  the  motor 
under  full  load  and  with  full  rated  voltage.  If  p  and  /  are  known, 
the  slip  may  be  calculated  as  explained  above. 

Various  methods  have  been  proposed  for  measuring  the  slip  of 
an  induction  motor  directly.  A  simple  piece  of  apparatus  for  this 
purpose  consists  of  a  black  disk  with  p  white  radial  lines  or  sectors 
painted  upon  it,  where  p  is  the  number  of  poles  of  the  motor.  This 
disk  is  attached  to  the  motor  pulley.  If,  when  the  motor  is  running, 
the  disk  is  illuminated  by  an  alternating-current  arc  lamp  supplied 
with  current  from  the  same  mains  as  the  motor,  the  white  lines  on 
the  disk  would  appear  stationary  if  the  motor  were  running  in 
synchronism.  If  the  motor  were  below  synchronism,  the  disk  would 
appear  to  rotate  with  a  speed  equal  to  the  difference  between  the 
synchronous  and  the  actual  speed.  Thus  the  slip  can  be  measured 
directly  by  counting  the  apparent  revolutions  per  minute  of  the  disk. 
For  example,  the  synchronous  speed  of  the  1-h.  p.  motor  con- 
sidered above,  is  1,200  r.  p.  m.  In  applying  the  above  test  the  disk 
made  70  apparent  revolutions  per  minute  when  the  motor  was 
running  at  full  load  and,  therefore,  the  slip  at  full-load  was 

'  or  5-83% 


416 


ALTERNATING-CURRENT  MACHINERY 


Performance  Curves  of  an  Induction  Motor.  Fig.  362  shows 
the  performance  curves  of  a  175-h.p.,  three-phase,  twelve-pole 
induction  motor  having  a  synchronous  speed  of  600  r.  p.  m.,  when 
supplied  with  three-phase  currents  at  550  volts  and  60  cycles  per 
second.  The  abscissas  of  all  the  curves  are  horse-power  output. 
The  dropping  of  all  the  curves  in  the  region  between  340  and  360 
horse-power  shows  that  the  maximum  power  which  the  machine 
can  develop  is  about  350  horse-power.  There  are  three  scales  of 
ordinates,  namely,  amperes  input,  per  cent,  and  pounds  torque. 

The  ordinates  of  the  curve  marked  "synchronism"  are  measured 
on  the  per  cent  scale,  and  they  represent  the  speeds  in  per  cent  of 
the  synchronous  speed  at  the  various  loads. 

The  ordinates  of  the  curve  marked  "amperes  input"  give  the 
amperes  input  ner  phase  at  the  various  loads. 


20     40     60     80     100     120    140    160    180  300  220  240  260  280  300  320  340 
Horse  F'oi^er    Output 

Fig.  362.     Performance  Curves  of  a  175-H.  P.  Three-Phase  Induction  Motor 

The  ordinates  of  the  curve  marked  "torque"  are  measured 
on  the  pounds  torque  scale,  and  they  represent  the  torque  developed 
by  the  motor  at  various  loads,  these  torques  being  expressed  in 
pounds  of  force  acting  at  one  foot  radius,  that  is,  in  pound-feet. 

The  ordinates  of  the  curves  marked  "efficiency",  "apparent 
efficiency",  and  "power  factor",  are  measured  on  the  per  cent  scale, 
and  they  represent  these  quantities  at  various  loads.  The  efficiency 

is,  of  course,  the  ratio, 


The  power  factor  is  the  factor  by 

which  the  product  of  volts  times  amperes  must  be  multiplied  to  give 
the  true  input  of  power  per  phase. 


ALTERNATING-CURRENT  MACHINERY  417 

The  product  volts  times  amperes  is  called  the  apparent  power 
delivered  by  an  alternating-current  generator,  and  the  apparent 

apparent  power  input 

efficiency  of  an  induction  motor  is  the  ratio,  -  — . 

output 

The  apparent  efficiency  is  thus  equal  to  the  true  efficiency  multiplied 
by  the  power  factor. 

The  relation  between  the  three  factors,  efficiency,  power  factor, 
and  apparent  efficiency,  will  be  made  clear  by  the  following  equa- 
tions, each  based  on  definition: 

useful  output 
efficiency  = 


power  factor  = 
apparent  efficiency  = 


actual  input 
actual  input 
apparent  input 
useful  output 


apparent  input 
From  the  above  three  equations,  it  is  evident  that 

apparent  efficiency  =  efficiency  X  power  factor 

When  the  output  of  a  motor  under  a  given  load  is  measured 
mechanically  by  observing  the  useful  torque  developed  at  the  arma- 
ture shaft,  and  the  speed  of  the  armature,  we  have 

27m'  T7  , 
useful  output  =  00         horse-power 


=  0.142  n'T  watts 

where  n'  is  the  speed  of  the  armature  in  revolutions  per  minute; 
and  T  is  the  useful  torque  in  pound-feet,  developed  by  the  armature. 
The  actual  input  to  a  three-phase  alternating-current  motor 
of  the  induction  type  may  be  expressed  as  follows: 

actual  input  =  V7  3   EIP  watts 

where  E  is  the  electromotive  force  (effective  value)  applied  between 
any  two  of  the  stator  terminals;  /  is  the  current  (effective  value)  in 
any  one  of  the  leads  supplying  the  stator  windings;  and  P  is  the  power 
factor  of  the  motor. 

The  apparent  input  to  a  three-phase  induction  motor  may  be 
expressed  as  follows: 

apparent  input  =  V  3   El  watts 


418  ALTERNATING-CURRENT  MACHINERY 

From  the  above  equations  we  may  easily  write  the  expressions  for 
the  efficiency,  power  factor,  and  apparent  efficiency,  as  follows : 

0.142  n'T 


v^TEIP 

power  factor  =  P  -  -  — / 

0.142wT 

apparent  efficiency  =      , —  „,. 
1     o  J-J*- 

Exartiple.     A  certain  12-pole  induction  motor  whose  performance  curves 
are  given  in  Fig.  362,  is  rated  as  follows: 

Number  of  phases,  3 

Output  rated  at  full  load,  175  horse-power 
Voltage  supplied  (between  mains)  550  volts 
Speed,  at  full  load,  585  r.p.m. 
Frequency,  cycles  per  second,  60 

A  brake  test  on  the  motor  when  running  at  full  load  gave  the  following 
data  (see  curves  in  Fig.  362). 

Useful  torque  on  rotor  shaft,  pound-feet.  1,560 

Speed,  in  revolutions  per  minute 585 

Amperes  input  per  phase 170 

Power  factor,  in  per  cent 88 

It  is  required  to  calculate  the  following  quantities  in  order  to  check 
the  accuracy  of  the  curves  plotted  in  Fig.  362. 

(a)  The  synchronous  speed  in  r.p.m. 

(b)  The  slip  at  full  load  in  per  cent. 

(c)  The  useful  output  in  watts  and  in  horse-power 

(d)  The  actual  input  in  watts 

(e)  The  efficiency 

(f)  The  apparent  input  in  watts 

(g)  The  apparent  efficiency 

Solution. 

(a)  The  synchronous  speed,  according  to  equation  (47)  is 

2/       2  X  60 

— — —  =  10  revolutions  per  second 

or 

10X60  =  600  r.p.m. 

(b)  The  slip  as  referred  to  on  page  415,  is 

100  =   (-     ^^-}  100  =  2.5  per  cent 


ALTERNATING-CURRENT  MACHINERY 


419 


TABLE  XI 
Capacities  of  Standard  Transformers 


H.  P. 

CAPACITY  MOTOR 

THREE-PHASE 

I 
TWO-PHASE 
2  TRANSFORMERS 

2  TRANSFORMERS 

3  TRANSFORMERS 

1 

.6kw. 

.5kw. 

.6kw. 

2 

1.5  kw. 

1.0  kw. 

1.0  kw. 

3 

2.0  kw. 

1.5  kw. 

1.5  kw. 

5 

3.0  kw. 

2.0  kw. 

3.0  kw. 

Ti 

4.0  kw. 

2.5  kw. 

4.0  kw. 

10 

5.0  kw. 

3.5  kw. 

5.0  kw. 

15 

7.5  kw. 

5.0  kw. 

7.5  kw. 

20 

10.0  kw. 

7.5  kw. 

10.0  kw. 

30 

15.0  kw. 

10.0  kw. 

15.0  kw. 

50 

25.0  kw. 

15.0  kw. 

25.0  kw. 

75 

25.0  kw. 

35.0  kw. 

100 

30.0  kw. 

45.0  kw. 

(c)     The  useful  output  from  the  above  equations  is  0.142  n'  T  watts. 
Substituting  the  given  values  of  nr  and  T,  we  have 

0.142  X  585  X  1560  =  129,600  watts 
or 


129,600 
746 


=  173  horse-power 


(d)  The  actual  input  in  watts  is  V  3     EIP,  whence  substituting  the 
given  values  of  E,  I,  and  P  gives 

actual  output  =  1.732  X  550  X  170  X  0.88 
=  142,500  watts 

(e)  The  efficiency  (real)  is 

useful  output       129,600 

-  =  • —       -  =  90.9  per  cent 
actual  input         142,500 

(f)  The  apparent  input  in  watts  is  V  3    El,  or 

1.732  X  550  X  170=  162,000  watts 

(g)  The  apparent  efficiency  is 

useful  output        129,600 

—  =  • —     — -  =  80  per  cent 
apparent  input        162,000  • 

A  comparison  of  the  above  results  with  the  corresponding 
values  obtained  from  the  curves  in  Fig.  362,  shows  on  the  whole 
a  very  satisfactory  agreement. 

The  power  factors  of  standard  commercial  induction  motors 
of  American  manufacture  vary  at  full  load  from  0.75  to  0.92,  de- 


420 


ALTERNATING-CURRENT  MACHINERY 


pending  upon  the  size  and  the  frequency  of  the  motor.  The  effi- 
ciencies range  from  0.80  to  0.95.  The  apparent  efficiencies  in  motors 
above  5  h.  p.  output  will  be  found,  as  a  rule,  not  less  than  0.75. 
This  means  that  the  transformers  supplying  current  to  induction 
motors  of  average  sizes,  must  have  an  aggregate  capacity  of  about 
one  kilowatt  for  every  horse-power  of  rated  output  of  the  motors. 

Table  XI  gives  approximate  capacities  of  standard  trans- 
formers that  should  be  used  with  two-phase  and  three-phase  induc- 
tion motors. 

SWITCHBOARD  AND  STATION  APPLIANCES 

The  switchboard  is  a  supporting  frame  upon  which  are  mounted 
most   of  the   measuring   instruments   and   safety   and  controlling 
devices    used   in  a  generating  or  receiving 
station. 

Lightning  arresters  are  usually  mounted 
on  the  wall  of  the  station  building  at  the 
points  where  the  lines  enter  the  building. 
Large  apparatus  such  as  rheostats,  oil- 
switches,  circuit  breakers,  and  voltage  regu- 
lators are  frequently  installed  at  a  distance 
from  the  switchboard,  and  are  controlled 
from  the  switchboard  by  systems  of  levers, 
by  compressed  air,  or  by  electrical  relays 
which  are  in  turn  controlled  by  contact 
switches  on  the  switchboard.  Thus  Fig.  363 
shows  a  large  rheostat  mounted  underneath 
the  floor  upon  which  the  switchboard  stands. 
The  rheostat  is  operated  by  means  of  sprocket-wheels  and  chain 
which  are  moved  by  a  hand-wheel  on  the  switchboard. 

SWITCHBOARDS 

Alternating-current  switchboards  differ  from  direct-current 
switchboards  as  follows : 

(a)  On  account  of  the  many  special  devices  such  as  frequency 
meters,  switchboard  transformers,  power-factor  indicators,  and 
synchronizing  devices,  which  are  used  in  alternating-current  work, 
and  are  not  used  in  direct-current  work. 


Fig.  363.  Diagram  Showing 

Switchboard  Control  for 

Large  Rheostats 


ALTERNATING-CURRENT  MACHINERY  421 

(b)  Because    of  complications  associated  with  the  use  of  an 
auxiliary  direct-current  generator  for  exciting  the  field  magnets  of 
alternators. 

(c)  Because   of  the  frequent  use  of  extremely  high  voltages, 
as  high  as  100,000  volts  or  more,  in  alternating-current  systems. 

Switchboards    are    now  usually  built   up   of  standard   panels^ 
uniform  in  size,  the  style  varying  with  the  service  required.    Large 
switchboards  for  handling  many  generators 
or  many  feeder  circuits,  are  built  up  by 
placing  a  number  of  these  standard  panels 
side   by  side.     This   method  of  building 
large  switchboards  has  the  following  ad- 
vantages : 

(1)  It  reduces  the  necessity  of  special 
work  to  a  minimum,  and  permits  the  use 
of  standard  apparatus,  thus  reducing  cost. 

(2)  It  provides  for  interchangeability 
of  panels,  thus  making  rearrangement  of 
feeder,  generator,  and  exciter  panels  easy 
and  convenient. 

(3)  The  use  of  standardpanels  uniformly 
equipped  with  standard  apparatus  makes 
it  easy  and  cheap  to  renew  damaged  parts. 

(4)  It  enables  extensions  to  be  made 
easily  and  systematically. 

In  some  large  stations  as  many  as  ten 
or  more  generator  panels  and  fifteen  or 
more  feeder  panels  are  erected  side  by 
side.  The  panels  of  a  switchboard  are 
usually  erected  and  wired  completely  at 
the  factory,  and  all  the  instruments  are 
attached.  After  thorough  inspection  and 
testing,  they  are  shipped  to  their  desti- 
nation, the  instruments  being  detached,  Fig  3~64  Standard  switchboard 
and  shipped  separately.  ''Ind^S^M?' 

Switchboards   are  usually  constructed 

of  a  skeleton  frame  of  angle  iron,  to  which  panels  of  marble  or 
slate  are  fastened  by  bolts  and  nuts.    The  various  instruments  are 


422          ALTERNATING-CURRENT  MACHINERY 

attached  to  the  marble  or  slate  panels/  In  many  cases  the  appara- 
tus itself  is  located  behind  the  board,  the  hand-wheel  or  operating 
lever  only,  being  placed  on  the  front  of  the  board. 

Slate,  when  entirely  free  from  metallic  veins,  is  a  fair  insulator, 
but  the  frequent  occurrence  of  such  veins,  and  the  tendency  of  slate 
to  absorb  moisture  from  the  air,  render  it  unreliable,  especially 
for  switchboards  on  which  high-voltage  apparatus  is  to  be  installed. 


0.PS.T.Q.C.JMTCH 
J-  CA/W  RECE/YER 

/1AME 
L  - 
M-SM4LL.  rW£  SLOCKS 


Fig.  365.     Front  Elevation  and  Connections  for  Board  Shown  in  Fig.  364 

Marble  is  the  standard  material  for  switchboard  panels,  and  it  only 
is  used  for  high-voltage  panels. 

Typical  Single=Phase  Switchboard.  Fig.  364  shows  a  front 
view  of  a  standard  switchboard  for  one  single-phase  generator  and 
two  feeder  circuits.  The  front  elevation  of  this  board  and  the  com- 
plete diagram  of  electrical  connections  are  shown  in  Fig.  365.  These 
switchboards  are  manufactured  by  the  Fort  Wayne  Electric  Works, 
for  electric  lighting  plants  of  moderate  size. 

The  standard  voltages  for  this  type  of  panel  are  1,150  and 
2,300  volts,  at  frequencies  of  from  25  to  140  cycles  per  second. 


ALTERNATING-CURRENT  MACHINERY     423 


The  kilowatt  capacity  or  rating  of  switchboard  panels  depends  on 

the  current  carrying  capacity  of  its  equipment.    Thus,  the  rating  of 

these  panels  ranges  from 

15  to  300  kilowatts  at 

2,300  volts  and  from  1\ 

to  150  kilowatts  at  1,150 

volts. 

The  ammeter  and 
voltmeter  used  on  these 
boards,  indeed  nearly 
all  switchboard  amme- 
ters and  voltmeters,  are 
of  the  electromagnetic 
type,  that  is,  they  con- 
sist of  a  coil  of  wire  ac- 
tuating a  movable  piece  of  soft  iron  to  which  the  pointer  is  at- 
tached. The  electromagnetic  type  is  described  on  page  67. 

The  essential  features  of  the  electrostatic  ground  detector  are 
also  described  on  page  67. 


Fig.  366.     Details  of  Switchboard  Fuse  Holder 


Generator  Sw.— S 


uses 
_Fje1dAmmete 

Voltmeter- 
A.C.Ann  metei A 

-Synchronizinq  Lamp — L 
Exciter  Rhe 

-Held  Switch 
Generator  Rhe. 
•Synch ronizincj  Plua 


}         Discharge 
Resistance" 


Generator 

Fig.  367.     Front  and  Side  Views  and  Complete  Diagram  of  Connections  for  Standard 
Panel  for  Two  or  More  Single-Phase  Alternators  in  Parallel 

The  two  double-pole  quick  air-break  generator  switches  with 
marble  barriers  are  shown  in  Fig.  364.    They  are  used  for  connect- 


424  ALTERNATING-CURRENT  MACHINERY 

ing  the  generator  terminals  to  either  or  both  of  the  feeder  circuits 
at  the  top  of  the  panel,  as  may  be  seen  in  the  diagram  of  connections 
shown  in  Fig.  365.  When  either  switch  is  opened,  the  arc  which  is 
produced  is  prevented  by  the  marble  barrier  from  flashing  across 
from  blade  to  blade  of  the  switch,  and  thus  short-circuiting  the 
generator. 

Instead  of  these  air-break  switches,  panels  may  be  equipped 
with  non-automatic  oil  switches,  mounted  at  the  back  of  the  board. 
The  type  of  switch  illustrated  in  Fig.  364,  and  which  breaks  the 
circuit  in  air  is  suitable  only  for  moderate  voltages  up  to  perhaps 
2,300  volts,  on  account  of  the  danger  of  arcing. 

One  of  the  fuse  holders,  shown  at  A  in  Fig.  365,  is  illustrated  in 
Fig.  366.  The  body  of  the  fuse  holder  consists  of  an  insulated  metallic 
chamber,  into  which  is  screwed  a  fiber  tube.  That  portion  of  the 
fuse  located  in  the  lower  chamber  has  a  smaller  cross-section  than 
the  portion  in  the  upper  tube,  which  insures  that  the  fuse  will  melt 
inside  the  chamber.  When  the  fuse  melts  and  breaks,  the  arc  is 
extinguished  by  the  explosive  action  which  accompanies  the  sudden 
heating  of  the  air  enclosed  in  the  fuse  chamber.  New  fuses  can 
be  easily  inserted  in  the  holder  by  removing  the  screw  plug  shown  at 
the  bottom  of  the  bulb.  The  holder  is  connected  by  means  of 
blades  fastened  to  each  end  which  fit  into  clips  mounted  at  the  top 
of  the  panel,  as  shown  in  Figs.  364  and  365.  The  potential  trans- 
former has  its  secondary  connected  through  a  high  resistance  to 
the  voltmeter.  If  the  ratio  of  transformation  of  the  potential  trans- 
former is  20  to  1,  and  the  voltmeter  reads  110  volts,  the  voltage  be- 
tween the  generator  terminals  is  20  X  110  =2,200  volts. 

Typical  Switchboard  for  Operating  Two  or  More  Single-Phase 
Alternators  in  Parallel.  Fig.  367  shows  front  and  side  elevations  and 
a  complete  diagram  of  electrical  connections  of  one  of  the  General 
Electric  Company's  standard  panels  for  one  of  several  single-phase 
generators  to  be  operated  in  parallel.  The  equipment  of  this 
panel  is  as  follows : 

1  double-pole  generator  switch,  mounted  on  the  back  of  the  board,  and 
operated  by  a  lever  on  the  front  of  the  board. 

2  expulsion  fuse  blocks  complete. 
1  generator  ammeter. 

1  field  ammeter. 

1  voltmeter  and  one  potential  transformer. 

1  field  switch. 


ALTERNATING-CURRENT  MACHINERY  425 

1  synchronizing  device,  complete. 

Tripod  and  front  plate  for  "generator  rheostat"  (in  the  field  circuit 
of  the  generator)  with  shaft  and  hand-wheel. 

Tripod  and  front  plate  for  "exciter  rheostat"  (in  the  field  circuit  of  the 
exciter.) 

All  necessary  framework  and  connections. 

The  location  of  apparatus  for  this  panel,  as  designated  by  letter, 
is  as  follows: 

R=  rheostat  in  the  field  circuit  of  the  generator,  "generator  rheostat." 
a  =  ammeter  in  the  field  circuit  of  the  generator  "field  ammeter." 

V  =  voltmeter  between  the  generator  terminals  (through  potential  trans- 
former). 

A  =  ammeter  for  the  main  alternating  current. 

L=  synchronizing  lamp. 

S  =  ' 'generator  switch"  in  the  main  circuit. 

r  =  rheostat  in  the  exciter  field  circuit,  "exciter  rheostat." 

This  apparatus  is  essentially  the  same  as  the  apparatus  already 
described  on  page  424  with  the  exception  of  the  ground. detector, 
the  field  switch  with  discharge  resistance,  and  the  synchronizing 
device.  One  ground  detector  is  sufficient  for  a  number  of  machines 
operated  in  parallel,  and  it  is  usually  mounted  on  a  bracket  attached 
to  one  of  the  generator  panels.  The  field  switch  is  arranged  to 
short-circuit  the  field  winding  of  the  alternator  at,  or  just  before, 
the  instant  of  disconnecting  the  exciter  from  the  field  windings. 
This  allows  the  current  in  the  field  winding  to  die  away  slowly. 
The  opening  of  a  field  switch  which  is  not  provided  with  a  resistance 
produces  an  excessively  high  electromotive  force  between  the  field 
terminals,  which  is  likely  to  cause  puncture  of  the  insulation  of  the 
field  windings. 

The  synchronizing  device  consists  of  the  synchronizing  bus 
bars  connecting  the  various  generator  panels;  the  synchronizing 
lamps;  a  small  transformer  (the  same  one  being  used  for  the  volt- 
meter) for  stepping  down  the  voltage  to  a  suitable  value  for  the 
synchronizing  lamps;  and  connection  plugs  for  connecting  the  sec- 
ondary of  the  potential  transformer  through  the  synchronizing 
lamps  to  the  synchronizing  busses.  Two  types  of  connecting  plugs 
are  used,  one  of  which  reverses  the  connections  made  by  the  other. 
The  complete  connections  of  the  synchronizing  device  for  two 
single-phase  machines  is  shown  in  Fig.  368. 

Polyphase  Switchboard.  In  case  of  polyphase  machines,  one 
phase  only  is  connected  to  the  synchronizing  device,  which  is,  there- 


426 


ALTERNATING-CURRENT  MACHINERY 


fore,  the  same  for  single-phase  and  for  polyphase  alternators.  The 
operation  of  alternators  in  parallel  is  very  common  in  modern  cen- 
tral stations.* 

The  voltmeters  and  synchronizing  device  are  always  connected 
back  of  the  main  generator  switch,  that  is,  between  the  generator 
and  the  switch,  for  the  reason  that  the  voltage  of  the  machine  must 
be  synchronized  before  the  main  switch  is  closed. 

The  connections  of  the  two  types  of  synchronizer  plugs  are 
shown  in  Fig.  368.  Neither  plug  is  used  when  one  alternator  only 
is  operated.  When  another  machine  is  to  be  put  into  operation, 


Synchronizing  Buses 


i 


ToBu&es 


Mafrt 
Switch  Q 


To  Buses 


Potential 
Transformer 


Mo/n 
Switch 


?i?k  A 


-:-h 


tift 


Potential 
Transformer) 


two  Types  of  P/ugs 
n,  General  -*""  *>"*"""*'''*>  To  <!«,»„,„ 

Fig.  368.     Complete  Connections  for  Synchronizing  Device  for  Two 
Single-Phase  Machines 

either  type  of  plug  is  used  to  connect  the  synchronizer  busses  to 
any  one  of  the  machines  already  running,  and  the  other  type  of 
plug  is  used  for  connecting  the  machine  which  is  being  synchronized 
to  the  synchronizer  busses.  Thus  the  synchronizer  busses  are 
oppositely  connected  to  the  two  machines,  and  the  synchronizing 
lamps  are  bright  when  the  conditions  are  proper  for  closing  the 
main  switch.  This  is  the  common  practice  of  the  General  Electric 
Company. 

When  more  than  two  generators  are  operated  in  parallel,  and 
one  direct-current  generator  is  used  as  an  exciter  for  supplying 
current  to  the  field  windings  of  all  the  alternators,  a  separate  switch- 
run  in  *^pd1«t«led. emanation  of  how  two  or  more  alternators  are  synchronized  in  order  to 
run  in  parallel  is  given  m  the  Appendix,  Part  VI,  page  462. 


ALTERNATING-CURRENT  MACHINERY 


427 


board  panel  called  the  exciter  panel,  is  usually  installed.  Upon 
this  panel  the  exciter  field  rheostat  and  controlling  devices  are 
mounted.  In  this  case  exciter  bus  bars  are  led  from  the  exciter 
panel  to  all  the  main  generator  panels. 

Typical  Switchboard  Panel  for  Two-Phase  Alternator  Operated 
in  Parallel  with  Other  Two-Phase  Machines.  Fig.  369  shows  front  and 
side  elevations,  and  complete  wiring  diagram  (back  view),  of  one  of 
the  General  Electric  Company's  switchboard  panels  for  a  two-phase 
alternator  which  is  to  be  operated  in  parallel  with  other  two-phase 
machines. 


-Fuses 

Exciter  Rheostat 
CAmmeters 
Synchronizinq  Lamp 
Field  Ammeter 
Voltmeter 
•Gen.  Rheostat 
Field  Switch 
"Synchronizinq  Pluq 

Generator  Switch 
"With  3  Barriers 


Discharqe 
Resistance  — 
of  Field  Switch 


;Exciter    Generator 


Fig.  369.     Front  and  Side  View  and  Complete  Wiring  Diagram  of  Switchboard  Panel  for 
Two-Phase  Alternator  Operated  in  Parallel  with  Other  Two-Phase  Machines 

The  potential  transformer  which  supplies  reduced  voltage  to 
the  voltmeter  and  to  the  synchronizer  busses  has  its  primary  con- 
nected across  one  phase  of  the  generator,  and  the  synchronizing  device 
is  exactly  the  same  as  for  the  single-phase  machines. 

The  equipment  of  this  two-phase  generator  panel  differs  from 
that  of  the  single-phase  panel  shown  in  Fig.  367,  in  the  following 
particulars : 

(a)  The  main  generator  switch  is  a  four-pole   switch  for  connecting 
the  four  leads  from  the  generator  to  the  four  .lines  which  pass  out  from  the 
top  of  the  board  through  four  fuses. 

(b)  Two  alternating-current  ammeters  are  used,  one  for  each  phase 


428 


ALTERNATING-CURRENT  MACHINERY 


The  "generator  rheostat"  is  in  the  field  circuit  of  the  two-phase  genera- 
tor, and  the  "exciter  rheostat"  is  in  the  field  circuit  of  the  exciter,  exactly 
as  in  Fig.  367. 

The  equipment  of  apparatus  for  this  panel  is  as  follows : 

R  =  rheostat  in  the  field  circuit  of  the  generator,  "generator  rheostat". 

a=ammeter  in  the  field  circuit  of  the  generator,  "field  ammeter". 

V  =  voltmeter  between  the  terminals  of  one  phase  of  the  generator 

(through  the  potential  transformer) . 

A  i&ndA  2  =  ammeters,  one  for  each  phase  of  the  generator. 
L=  synchronizing  lamp. 

r  =  rheostat  in  the  field  circuit  of  the  exciter,  "exciter  rheostat". 
/,/,/,/  =  ^ses. 

The  four  small  circles  above  and  below  the  words  "synchronizing  busses" 
are  the  points  of  ohe  four-pole  main  switch. 


5Y/V   A          A-SX/V 
LAMP^S          ^PLUG 


Fig.  370.     Complete  Wiring  Diagram  for  Two  Two-Phase  Generators  Running  in  Parallel 

Fig.  370  shows  the  complete  connections  of  two  two-phase 
generators  for  parallel  running. 

Typical  Switchboard  Panel  for  Three-Phase  Generator  Operated 
in  Parallel  with  Other  Three-Phase  Machines.  Fig.  371  shows  front 
and  side  elevations,  and  complete  wiring  diagram  (back  view)  of  one 
of  the  General  Electric  Company's  switchboard  panels  for  a  three- 
phase  alternator,  which  is  to  be  operated  in  parallel  with  other 
three-phase  machines.  The  potential  transformer  which  supplies 
reduced  voltage  to  the  voltmeter  and  to  the  synchronizing  busses 


ALTERNATING-CURRENT  MACHINERY 


429 


has  its  primary  connected  across  one  phase  of  the  generator,  and 
the  synchronizing  device  is  exactly  the  same  as  for  single-phase 
machines.  i 

The  equipment  of  this  three-phase  panel  differs  from  that  of 
the  single-phase  and  two-phase  panels  shown  in  Figs.  367  and  369 
in  that  (a)  the  main  generator  switch  is  a  three-pole  switch  for  con- 
necting the  three  generator  leads  to  the  three  lines  which  pass  out 
from  the  top  of  the  board  through  three  fuses;  and  (b)  three  alter- 
nating-current ammeters  are  used,  one  for  each  phase. 

Fig.  371  also  shows  the  following  points,  which  however,  are 
not  characteristic  of  a  thiee-phase  panel,  but  might  be  used  on  a 
two-phase  panel. 


A  C  Ammeters 
A.  C.  Voltmeter 
O.C.Ammeter  (Field) 
Synchronizing  Lamp 
Generator  Field  Rheostat 
eld    Switch 

Voltmeter  Plug  Switch 
Synchronizing  Rug  5/vitch 

•Generator  Switch 


I  Bus  Bars 


Fig.  371.     Front  and  Side  View  and  Complete  Wiring  Diagram  for  a  Three-Phase 
Generator  Operated  in  Parallel  with  Other  Three-Phase  Machines 

(a)  The  two  phases  not  connected  to  the  potential  transformer  proper, 
are  stepped-down  to  a  reduced  voltage  by  the  use  of  one  additional  potential 
transformer  T,  Fig.  371. 

As  explained  on  page  260,  two  transformers  suffice  for  stepping-down 
three  phases  to  two  phases.  Both  potential  transformers  have  their  second- 
aries connected  to  a  "voltmeter  plug  switch",  by  means  of  which  the  volt- 
meter may  be  connected  so  as  to  indicate,  at  the  will  of  the  operator,  the 
voltage  of  any  one  of  the  three  phases. 

(b)  The  lines  which  pass  out  at  the  top  of  the  panel  in  Fig.  371  are 
shown  connected  to  the  three  main  bus  bars  or  rods. 

(c)  No  exciter  is  shown  in  Fig.  371,  and  no  exciter  field  rheostat,  but 
the  panel  is  arranged  so  that  one  large  exciter  may  be  used  for  all  the  gener- 
ators.    For  this  purpose  exciter  busses  (two  of  them),  connect  the  one  large 
exciter  to  all  the  generator  panels. 

(d)  The  generator  field  rheostat  is  geared  to  the  hand  wheel  by  means 
of  sprocket  wheels  and  chain  on  the  back  of  the  switchboard. 


430 


ALTERNATING-CURRENT  MACHINERY 


Ai,  A: 


The  equipment  of  apparatus  for  this  panel  is  as  follows: 

R  =  "generator  field  rheostat". 

a  =  ammeter  (direct-current)  in  the  field  of  the  generator. 

V= voltmeter  between  the  terminals  of  one  phase  of  the  generator 

(through  potential  transformer). 
p=  voltmeter  plug  switch. 

A3=  ammeters  for  alternating  currents,  one  in  each  phase  of  the  gen- 
erator. 

L  =  synchronizing  lamp. 
£  =  maiil  generator  switch. 
/,/,/= fuses: 

T  =  additional  potential  transformer. 

Feeder  Panels.     In  large  generating  stations,  the  bus  bars, 
to  which  the  various  generators  are  connected  in  parallel,  lead  from 

the  generator  panels  to  the  feeder  panels. 
The  feeders  are  the  separate  and  distinct 
circuits  which  receive  current  from  the 
bus  bars  and  transmit  it  outside  the 
station  to  points  more  or  less  remote. 
Each  pair  of  feeders  (or  set  of  three  or 
four  in  polyphase  distribution),  as  it 
comes  into  the  station,  is  protected  by 
lightning  arresters.  From  the  lightning 
arresters  the  feeders  are  brought  to  the 
feeder  panels  through  fuse  blocks,  and 
through  ammeters,  and  connected  to  bus 
bars  by  means  of  suitable  switches. 

When  there  are  but  few  feeder  cir- 
cuits, the  feeder  switches  are  mounted  on 
the  generator  panel  as  shown  in  Fig.  364. 
Ground  detectors  are  used  primarily 
for  detecting  grounds  on  feeder  circuits, 
and  where  many  feeders  are  connected 

to  the  bus  bars,  tne  ground  detectors  are  mounted  on  the  feeder 
panels.  When  there  are  but  few  feeder  circuits  the  ground  detector 
may  be  connected  to  the  bus  bars,  in  which  case  the  ground 
detector  may  be  mounted  on  a  generator  panel  or  on  a  feeder  panel, 
if  feeder  panels  are  used. 

It  is  frequently  desirable  to  control  the  voltage  supplied  to  a 
feeder  circuit  independently  of  the  voltage  between  the  bus  bars. 


Fig.  372.     Front  Elevation  of 

Typical  Three-Phase 

Feeder  Panel 


ALTERNATING-CURRENT  MACHINERY 


431 


For  this  purpose  voltage  or  potential  regulators  are  used,  and  these 
voltage  regulators  are  either  mounted  upon  the  feeder  panels  or  are 
controlled  by  levers  or  hand  wheels  which  are  mounted  on  the 
feeder  panels. 


*         Fig.  373.     Front  View  of  Standard  High  Voltage  Switchboard 
for  Central  Stations,  etc. 

When  the  energy  (watt-hours)  delivered  to  a  feeder  circuit  is 
to  be  measured,  the  integrating  watt-hour  meter  is  mounted  on 
the  feeder  panel. 


Fig.  374.     Rear  View  of  Switchboard  Shown  in  Fig.  373 

Circuit  breakers,  when  used,  are  usually  mounted  upon  the 
feeder  panels  and  arranged  to  open  the  feeder  switch  when  the  cur- 
rent delivered  to  the  feeder  becomes  excessive. 


432 


ALTERNATING-CURRENT  MACHINERY 


ALTERNATING-CURRENT  MACHINERY  433 

Fig.  372  shows  a  front  elevation  of  a  typical  three-phase  feeder 
panel  manufactured  by  the  Crocker- Wheeler  Company.  The  equip- 
ment of  this  panel  includes  the  following  apparatus: 

3  alternating-current  ammeters,  one  for  each  line  of  the  three-line  feeder. 
Necessary  current  transformers. 

1  three-pole,  single-throw,  non-automatic  oil  switch,  mounted  back  of 
the  panel  and  operated  by  a  lever  on  the  front  of  the  panel. 
3  expulsion  fuse  blocks. 

High= Voltage  Panels.  In  alternating-current  generating  stations 
for  long-distance  transmission,  the  alternating  currents  are  generated 
at  a  medium  or  low  voltage,  and  are  then  stepped-up  to  10,000  or 
to  100,000  volts  or  more  for  transmission.  In  such  stations,  the  low- 
voltage  switches  and  controlling  devices,  including  exciter  switches 
and  rheostats,  are  mounted  on  panels  separate  from  the  high-voltage 
switches  and  devices.  Such  stations  have,  therefore,  low-voltage 
panels  and  high- voltage  panels.  The  high- voltage  panels  differ  from 
the  low-voltage  panels  in  having  very  much  greater  distances  be- 
tween the  high-voltage  parts,  in  order  to  avoid  the  danger  of  short- 
circuit  by  sparking  across  through  the  air,  and  in  having  special 
forms  of  remote  control  switches. 

Figs.  373  to  375  give  views  of  a  standard  switchboard  designed 
for  central  stations  and  industrial  plants  for  voltages  from  2,200 
to  13,000.  They  are  given  here  to  illustrate  especially  the  so-called 
"remote  control"  method  of  operating  switches. 

SPECIAL  SWITCHBOARD  APPARATUS 

Lincoln  Synchronizer.  When  incandescent  lamps  are  used  as 
synchronism  indicators  in  the  starting  of  a  synchronous  motor,  or 
in  the  paralleling  of  alternators,  the  pulsations  of  brightness  indicate 
only  the  difference  in  frequency  of  the  two  machines,  and  it  is  in 
general  impossible  to  tell  from  the  behavior  of  the  lamps  which  of 
the  machines  is  running  at  the  greater  frequency. 

The  Lincoln  synchronizer  is  a  device  for  indicating  positively 
the  difference  in  the  frequency  of  two  alternators  which  are  being 
adjusted  into  synchronism,  and  also  for  indicating  positively  the 
phase  difference  of  the  two  machines  at  each  instant. 

The  Lincoln  synchronizer  is  a  very  small  machine  of  which 
the  iron  parts  are  exactly  like  the  field  magnet  and  armature  core 


434  ALTERNATING-CURRENT  MACHINERY 


of  a  very  small  two-pole  direct-current  motor,  except  that  both 
field  magnet  and  armature  core  are  laminated.    The  field  magnet 

is  excited  with  alternating  cur- 
rent by  connecting  the  field  wind- 
ing to  the  terminals  of  A,  of  the 
alternators  A  and  E,  which  are 
being  synchronized.  The  arma- 
ture of  the  synchronizer  is  wound 
with  two  coils  c  and  d  at  right 
angles  to  each  other.  Each  of 
these  coils  is  independently  con- 
nected through  collecting  rings 
to  the  terminals  of  alternator  B. 
Coil  c  has  connected  in  .  series 
with  it  a  non-inductive  resist- 
ance, and  coil  d  has  connected 

ljl    S6rieS  wlth    'lt    a    lai>Se  induCl> 

ance.  The  armature  of  the  small 
machine  has  attached  to  -  it  a  pointer  which  moves  over  a  divided 
circle.  When  machines  A  and  B  have  exactly  the  same  frequency, 
this  pointer  is  stationary  and  its  reading  on  the  circle  indicates  the 
phase  difference  between  the  two  machines.  When  machine  A  is 


Fig.  376.-  Lincoln  Synchroscope 


&£/S  BAffS 

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TffAMSfOffrtL-ff 

VOL 
TffA 

A 

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WA/MSW 

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vo 

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A 

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WA 
<.TAG£          c 

\  \ 

//YSW, 

\ 

<TCH 

(wt>e»vjfd)\      \-SCOPE} 

WWW  
WWW. 

^T 

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WWWl 

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GfM/y 

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TO 

GEN.NL 

e 

Fig.  377. 


Wiring  Diagram  for  a  Synchrosc9pe  for  Single-Phase  or  One  Phase  of  Poly- 
phase Circuit 


running  at  a  slightly  greater  frequency  than  B,  the  pointer  rotates  in 
a  certain  direction  indicating  at  each  instant  the  changing  phase- 
difference  between  A  and  B.  When  machine  A  is  running  at  a 


ALTERNATING-CURRENT  MACHINERY  435 

slightly  lower  frequency  than  B,  the  pointer  rotates  in  the  oppo- 
site direction,  indicating  at  each  instant  the  changing  phase-differ- 
ence between  A  and  B.  The  speed  of  the  pointer  in  revolutions  per 
second  is  in  each  case  equal  to  the  difference  in  frequency  of 
machines  A  and  B}  in  cycles  per  second. 

Fig.  376  is  a  view  of  the  exterior  of  a  Lincoln  synchroscope, 
"type  A,"  as  made  by  the  Westinghouse  Electric  Company.  It  has  a 
9-inch  dial  and  is  intended  to  be  mounted  on  the  switchboard.  Its 
coils  are  wound  for  100  volts  so  that  a  voltage  transformer  is  neces- 
sary for  connecting  it  to  the  bus  bars,  and  one  voltage  transformer 
for  each  machine  to  be  synchronized. 

Fig.  377  is  a  diagram  of  connections  for  the  "type  A"  synchro- 
scope for  single-phase  or  one  phase  of  polyphase  circuits  for  voltages 
of  500  and  over. 

CIRCUIT=INTERRUPTINQ  DEVICES 

Devices  whose  sole  function  is  the  opening  of  an  electric  circuit 
may  be  classified  into:  fuses,  switches,  and  circuit  breakers.  In 
the  design  of  any  circuit-interrupting  device  four  general  require- 
ments are  involved: 

(a)  To  provide  means  for  carrying  the  rated  current  of  the  device 
without  excessive  drop  of  voltage  or  heating,  and  also  such  overloads  as  will 
occur  in  practice. 

(b)  To  insulate  all  live  parts  for  the  maximum  voltage. 

(c)  To  provide  mechanical  means  for  opening  the  circuit. 

(d)  To  prevent  or  make  harmless  any  arc  that  may  form. 

Circuit-interrupting  devices  may  be  automatic  or  non-auto- 
matic. In  general,  fuses  are  always  automatic,  switches  always 
non-automatic,  and  circuit  breakers  either  one  or  the  other.  In 
general,  circuit  breakers  are  so  arranged  that  a  spring  or  gravity 
tends  to  open  them,  and  they  are  held  closed  by  latches  or  toggle 
mechanisms  It  is  this  feature  which  makes  the  difference  between 
a  switch  and  a  non-automatic  circuit  breaker. 

Fuses.  A  fuse  is  the  simplest  and  cheapest  circuit-breaking 
device.  It  consists  of  a  wire  or  link  of  fusible  metal  (usually  of  lead 
alloyed  with  zinc  and  tin)  connected  electrically  in  series  with  the 
circuit  to  be  protected.  When  more  than  the  predetermined  current 
passes  through  it,  the  heat  generated  in  it  due  to  its  resistance  is 
sufficient  to  melt  it,  and  the  fuse  is  said  to  "blow",  thus  opening  the 
circuit.  The  open  or  exposed  type  of  wire-  or  link-fuse  is  rarely  used 


436  ALTERNATING-CURRENT  MACHINERY 

on  account  of  its  tendency  to  scatter  molten  metal  and  increase 
the  fire  hazard.  Moreover,  open  link  fuses  cannot  be  accurately 
rated  on  account  of  the  exposure  and  variable  length  of  the  fusible 
strip.  It  was  to  overcome  these  objections  that  the  enclosed  type 
of  fuse  was  developed.  The  latter  type  of  fuse*  consists  of  a  fusible 
wire  or  link  enclosed  within  a  fiber  tube  fitted  with  a  fireproof  material 
to  exclude  the  air  and  suppress  the  arc  formed  when  the  fuse  em- 
bedded in  this  material  melts  and  opens  the  circuit.  Suitable  ter- 
minals are  provided  so  that  the  fuse  may  be  mounted  in  a  fuse  block. 
Enclosed  fuses  have  been  standardized  into  classes  according  to  the 
voltage  and  ampere  capacity,  and  are  rated  so  that  they  will  carry 
ten  per  cent  overload  indefinitely,  and  will  open  at  twenty-five  pfcr 
cent  overload. 

A  very  important  difference  between  the  fuse  and  the  circuit 
breaker  is  the  matter  of  time  element.  Thus  the  over-load  circuit 
breaker  depends  for  its  operation  on  the  value  of  the  current  only, 
whereas  the  blowing  of  a  fuse  depends  not  only  on  the  current  but 
also  on  the  time  it  flows.  That  is,  the  breaker  will  open  immediately 
atTany  load  in  excess  of  that  for  which  it  is  set,  and  will  not  operate 
at  any  smaller  current,  no  matter  how  long  it  may  continue.  Stand- 
ard fuses,  on  the  other  hand,  will  in  time  operate  at  an  overload  of 
only  25  per  cent,  and  will  open  in  a  proportionately  shorter  time  with 
greater  over  loads.  Certain  conditions,  therefore,  require  the  use  of 
fuses,  while  others  are  better  met  by  breakers,  or  by  a  combination 
of  the  two.  -  —  .  - 

Air=Break  Switches.  Special  styles  of  switches  are  required  in 
many  cases  in  alternating-current  work  on  account  of  the  excessively 
high  voltages,  especially  when  the  switch  has  to  handle  large  currents 
at  high  voltage. 

Mention  has  been  made  on  page  421  of  the  use  of  a  marble  slab 
between  the  blades  and  contact  points  of  a  double-pole  switch  to 
prevent  the  arcs,  formed  when  the  switch  is  opened,  from  flashing 
across  and  short-circuiting  the  points  of  the  switch  which  are  con- 
nected to  the  generator  terminals.  Such  a  switch  with  marble 
barrier  is  shown  in  Fig.  364. 

Arcing  is  reduced  to  a  minimum  in  every  case  by  opening  the 
switch  very  quickly,  and  all  of  the'special  alternating-current  switches 

Evatem^if/^K8^11  type  °fonU8e'-  much  used  for  medium  voltages  in  alternating-current 
systems,  is  described  on  page  305,  m  connection  with  transformer  fuse  blocks. 


ALTERNATING-CURRENT  MACHINERY 


437 


described  below,  are  arranged  to  open  with  a  snap,,  even  though  the 
controlling  lever  is  moved  slowly. 

Fig.  378  shows  a  common  form  of  knife  switch  specially  con- 
structed to  give  a  quick  break.  The  switch  blade  is  in  two  parts 
a  and  b,  to  one  of  which,  a,  is  attached  the  operating  handle.  The 
two  parts  a  and  b  of  the  blade  are  connected  by  strong  helical  springs 


Fig.  378.     "Quick  Break"  Knife  Switches 

s  s,  one  on  each  side  of  the  blade.  When  the  handle  is  pulled  forward 
to  open  the  switch,  blade  a  leaves  clip  c,  while  the  springs  are  put 
in  tension  more  and  more  until  the  switch  blade  b  is  pulled  from  the 
clip  c,  with  a  snap,  thus  causing  a  sudden  breaking  of  the  circuit. 
Fig.  378  shows  two  single-pole  quick-break  switches.  The  one  on 
the  left,  designed  for  600  amperes,  is  provided  with  two  massive 
tubular  terminals  1 1,  into  which  the  terminals  of  the  main  circuit  are 
inserted  and  soldered.  The  switch  on  the  right,  designed  to  carry 
3,600  amperes,  is  furnished  with  massive  studs  d  d,  which  project 
through  holes  drilled  in  the  switchboard  to  the  back  of  the  board. 


438  ALTERNATING-CURRENT  MACHINERY 

Connection  to  these  studs  is  made  by  clamping  the  terminals  of  the 
main  circuit  to  them  by  means  of  the  threaded  nuts  n  n.  On  account 
of  the  large  current^to  be  carried,  extra  large  contact  surfaces  are 
provided  between  switch  blades  and  clips,  as  seen  in  the  figure. 

Damage  to  switch-blades  and  clips,  due  to  arcing,  is  reduced 
to  a  minimum  by  providing  carbon  blocks  which  make  a  connection 
in  parallel  with  the  switch  connection  proper.  When  the  switch  is 
opened,  the  metal  connection  is  broken  first,  and  the  carbon  connec- 
tions afterwards,  so  that  the  arcing  always  takes  place  between  the 
carbon  blocks. 


Fig.  379.     Half  Section  of  Over-Load  Type  of  Circuit  Breaker — Closed 

Circuit  Breakers.  In  Fig.  379  there  is  shown  in  elevation  and 
half  section  a  plain  overload  type  of  circuit  breaker  for  either  direct- 
.or  alternating-current  circuits  up  to  600  volts.  Fig.  380  is  a  general 
view  of  the  same  single  pole  breaker  as  manufactured  by  the  Rclkr- 
Smith  Company.  Fig.  379  shows  the  breaker  closed  and  Fig.  380, 
open.  Referring  to  Fig.  379,  A  is  a  rectangular  magnet  core  journaled 
on  the  cylindrical  shaft  shown,  and  supported  by  it  between  two 
non-magnetic  supporting  frames,  only  one  of  which,  B,  is  shown  in 
the  sectional  view.  To  A  is  secured  one  of  the  terminals  of  the  main 


ALTERNATING-CURRENT  MACHINERY 


439 


coil  C,  which  is  made  up  of  a  number  of  hard-rolled  copper  strips; 
and  also  the  arm  D.  The  other  terminal  of  the  laminated  winding 
is  secured  to  the  lower  stud  P.  Current  entering  through  the  stud 
P  passes  through  the  main  windings  C  into  the  arm  D,  through  the 
contact  plate  E  into  the  stationary  copper  brush  F,  and  finally  out 
through  the  upper  stud  Q.  When  the  current  through  the  breakei 
exceeds  the  predetermined  value  for  which  it  is  adjusted,  the  attrac- 
tion exerted  by  the  core  A  on  the  ends  K  of  the  pivoted  armature 
causes  it  to  rise  with  increasing  speed  until  the  finger  M,  a  part  of 
the  armature,  strikes  R.  The  roller  H  is  thus  forced  downward 
past  the  roller  G,  thus  permitting 
the  strong  outward  pressure  of 
the  brush  F  and  that  of  the  coil 
C  to  throw  the  arm  outward  and 
thus  break  the  circuit,  first  be- 
tween the  brush  fingers  and  the 
contact  plate  E,  and  finally  be- 
tween the  carbons  S  and  F.  It 
will  be  noted  that  the  carbon 
block  F  is  mounted  on  a  spring 
support  and  thus  maintains  its 
contact  with  S  after  the  main 
contacts  between  the  copper 
brush  F  and  plate  E  have  been 
broken.  This  action  effectually 
prevents  arcing  and  pitting  of 
the  main  contacts  which  would 
soon  wear  out  the  breaker. 

To  reset  the  breaker,  the  handle  which  the  act  of  opening  has 
raised,  is  pulled  down,  thus  forcing  the  roller  H  up  past  the  roller 
G,  which  compresses  the  brush  F  and  the  coil  C,  by  forcing  the 
arm  back  into  its  initial  position. 

\^_  These  breakers  are  also  made  with  under-load,  no-voltage, 
and  shunt-trip  attachments,  and  may  be  operated  from  any  distant 
point  if  desired. 

The  same  methods  are  available  for  suppressing  the  arc  be- 
tween the  break  points  of  a  switch  as  for  suppressing  the  arc  which 
tends  to  maintain  itself  between  the  terminals  of  a  fuse  link. when 


Fig.  380.     General  View  of  Roller-Smith' 
Single-Pole  Breaker — Open 


440 


ALTERNATING-CURRENT  MACHINERY 


the  link  fuses.  Thus  some  of  the  older  switches  were  designed  to 
break  the  circuit  between  the  end  of  a  movable  copper  rod  and  a  sta- 
tionary copper  socket,  both  of  which  are  surrounded  by  a  porcelain 
tube,  thus  utilizing  the  expulsion  principle  for  suppressing  the  arc. 
The  most  effective  method,  however,  for  suppressing  the  arc* 
on  a  high-voltage  switch  is  to  design  the  switch  so  as  to  open  quickly 
under  oil,  and  for  very  high  voltages  to  provide  for  several  simul- 
taneous breaks  in  series. 


Fig.  381.     General  Electric  Oil  Break 
Switch  Mounted  on  Panel 


Fig.  382.     General  Electric  Oil  Break  Switch 
— Rear  View — Switch  Open 


Oil=Break  Switches.  Air-break  switches  are  not  now  considered 
reliable  or  safe  for  alternating-current  circuits  above  about  600  volts, 
hence  the  adoption  of  oil-break  switches  for  practically  all  alter- 
nating-current circuits  of  440  volts  and  over  is  today  almost  universal. 

Figs.  381  and  382  are  views  of  a  typical  General  Electric  oil- 
break  switch,  mounted  on  a  panel.  The  six  insulators  which  support 


ALTERNATING-CURRENT  MACHINERY 


441 


the  stationary  contacts  and  studs  each  consist  of  a  single  piece  of 
porcelain,  and  can  be  easily  detached  from  the  frame  when  necessary. 
The  contact  fingers  are  flared,  are  of  copper,  and  are  fastened  to  the 
studs  by  heavy  springs  which  insure  good  contact  with  the  moving 
blades.  The  oil  tanks  are  of  sheet  metal  lined  with  maple,  and  are  de- 
signed to  be  easily  removable  for  inspection  of  switch  contacts  and  oil. 
These  oil  switches  are  designed  with  operating  mechanism 
mounted  directly  on  switchboard  panel  and  the  oil  switch  proper 
mounted : 


Fig.  383.     Westinghouse  Three-Pole  Oil  Circuit  Breaker— "Remote  Control" 

(a)  directly  on  panel; 

(b)  on  pipe  framework  directly  back  of  panel; 

(c)  on  pipe  framework  remote  from  panel;  or 

(d)  on  flat  surfaces  or  in  cells  remote  from  panel. 

These  switches  are  made  single-throw  with  one,  two,  three,  or 
four  poles;  and  for  voltages  of  2,500  up  to  15,000,  and  with  current 
capacities  ranging  from  1,000  to  300  amperes,  respectively. 

Any  of  these  oil  switches  may  have  operating  mechanisms 
which  are  either  non-automatic,  or  automatic  with  one,  two,  or  three 
overload-coil  attachments.  Fig.  381  shows  front  view  of  the  switch 
closed,  and  Fig.  382  back  view  of  the  switch  open. 


442  ALTERNATING-CURRENT  MACHINERY 

Great  improvement  has  been  made  in  recent  years  in  the  design 
of  large  circuit  breakers  for  protecting  alternating-current  circuits 
up  to  110,000  volts  and  over.  Fig.  383  shows  a  Westinghouse  three- 
pole  oil  circuit-breaker  for  44,000-volt  circuits.  Its  capacity  is  300 
amperes.  It  is  hand-operated  and  illustrates  what  is  called  "remote- 
control".  This  type  of  breaker  may  also  be  electrically  operated 


Fig.  384. 


"Type  GA"  Westinghouse  Oil  Circuit  Breaker  Removed  from  Tank  Showing  Open 
and  Closed  Positions 


by  means  of  suitable  solenoids  or  coils  energized  by  direct  current 
under  the  control  of  the  operator.  They  may  also  be  made  automatic 
in  action  by  the  addition  of  suitable  relay  attachments. 

Fig.  384  shows  a  single-pole  of  the  "type  GA"  Westinghouse  oil 
circuit  breaker  removed  from  its  tank,  in  both  the  open  and  closed 
positions.  The  upper  or  fixed  contacts  are  firmly  secured  to  the 
lower  end  of  the  leads,  which  are  clamped  by  iron  collars,  which  in 


ALTERNATING-CURRENT  MACHINERY 


443 


turn  are  bolted  to  the  tank  cover  through  which  the  leads  pass. 
This  upper  contact  consists  of  a  circular  piece  of  brass  of  greater 
area  than  the  moving  contact,  which  insures  the  entire  surface  of  the 
latter  bearing  upon  the  stationary  contact,  the  object  being  to 
eliminate  the  necessity  of  accurately  centering  the  contacts  one 
upon  the  other. 

The  lower  or  movable  contacts  are  carried  by  a  heavy  metallic 
cross  bar  and  consist  of  pieces  of  cylindrical  brass  rod  backed  by 
compression  springs  of  sufficient  strength  to  insure  good  contact 
and  which  also  render  the  contacts  self-aligning.  Copper  braid 
shunts  are  used  to  carry  the  current 
around  the  springs  in  order  to  pre- 
vent any  deterioration  by  the  pas- 
sage of  current  through  them. 

It  will  thus  be  seen  that  the 
contact  is  of  the  simple  "butt"  type 
between  two  circular  plane  surfaces. 
The  high  voltage  necessitates  the 
unusually  long,  widely  separated, 
and  specially  irrsulated  "condenser" 
type  of  terminals  shown  in  Fig.  384. 

The  general  type  of  oil  switch 
illustrated  in  Fig.  383  is  today  the 
standard  for  high-voltage  circuits 
carrying  large  alternating  currents, 
and  is  used  extensively  in  many  of 
the  most  recent  high-voltage  gen- 
erating stations.  . 

Fig.  385  is  a  general  view  of 
a  General  Electric  three-pole  oil 
switch,  or  rather  three  single-pole  switches  operated  by  an  electric 
motor,  which  is  controlled  from  the  switchboard.  Each  single- 
pole  switch  in  Fig.  385  is  mounted  in  a  separate  brick  compart- 
ment. The  oil  is  contained  in  long  metal  cylinders  in  order  to  reduce 
the  amount  of  oil  to  a  minimum.  The  long  brass  connecting  rods 
pass  through  holes  in  the  porcelain  bushings  in  the  tops  of  the  cylinders 
and  the  ends  of  these  rods  carry  conical  lugs  which  fit  into  spring 
sockets.  These  spring  sockets  are  connected  to  rods  which  pass 


Fig.  385.    General  Electric  Three-Pole  Oil 
Switch  Operated  by  Electric  Motor 


444         ALTERNATING-CURRENT  MACHINERY 


out  through  porcelain  bushings  in  the  bottoms  of  the  cylinders,  and 
these  lower  rods  are  connected  together  in  each  compartment,  thus 
making  a  double-break  single-pole  switch.  The  metal  cylinders  in 

Fig.  385  are  entirely  insulated  from  the 
switch  points  proper. 

Feeder  or  Voltage  Regulators.  When 
several  feeder  circuits  are  supplied  from 
the  same  bus  bars,  and  when  it  is  de- 
sired to  control  the  voltage  on  each 
feeder  circuit  independently  of  the 
others,  feeder  regulators  are  required. 
Single-phase  feeder  regulators  are 
autotransformers  with  their  primary 
coils  connected  across  (that  is,  as  a  shunt 
to)  the  bus  bars,  and  their  secondaries 
connected  in  series  with  the  feeder  cir- 
cuit. They  are  of  three  types,  as  follows: 


Fig.  386.     Stillwell  Single-Phase 
VoltageRegulator  Complete 


(a)  A  type  in  which  the  secondary  coil 
has  many  leads  brought  out  to  points  on  a 
dial  switch,  so  that  the  number  of  active 
turns  on  the  secondary  coil  may  be  changed  at 


will,  thus  permitting  the  adjustment  of  the  feeder  voltage  to  any  desired  value, 
(b)     A  type  in  which  the  primary  and  secondary  coils  are  wound  at 

right-angles  to  each  other  on  the  inner  face  of  a  laminated  iron  ring  very  much 

like  the  stator  ring  of  an  induction 
motor.  The  magnetic  flux,  due  to 
the  primary  coil,  is  made  to  pass  in 
whole  or  in  part  through  the  secon- 
dary coil  by  turning  a  laminated  core 
as  explained  below. 

(c)  Polyphase  feeder-regulators 
usually  consist  of  several  single-phase 
regulators,  one  for  each  phase.  The 
advantage  of  this  arrangement  is 
that  the  voltage  of  each  phase  may 
be  controlled  separately.  A  com- 
bined poly-phase  feeder  regulator  is, 
however,  sometimes  used. 

(a)     Stillwell  Regulator.    Fig. 

Fig.  387.     Dial  of  Stillwell  Single-Phase  oo~    .  l       •  i>        o^ll 

Voltage  Regulator  386  is  a  general  vie.w  or  a  btill- 

well  single-phase  voltage  regula- 
The  dial  switch  alone  is  shown  in 


tor  with  its  dial  switch  complete. 


ALTERNATING-CURRENT  MACHINERY         445 


Fig.  387.  The  complete  internal  connections  are  shown  in  Fig.  388. 
The  primary  coil  of  the  regulator  is  permanently  connected  across  the 
bus  bars  (or  generator  terminals). 
One  feeder  wire  passes  out  directly 
from  the  generator  and  the  other 
passes  through  few,  or  many,  turns 
of  the  secondary  coil  of  the  regulator, 
and  thence  to  the  line.  The  re- 
versing switch  A\  serves  to  connect 
the  feeder  /  to  one  or  the  other  ter- 
minal of  the  secondary  coil,  and  the 
arm  of  the  dial  switch  connects  the 
line  wire  to  any  one  of  the  taps 
which  are  brought  out  from  the 
secondary  coil.  For  one  position  of 
the  reversing  switch,  the  induced 
voltage  in  the  secondary  turns,  which 
are  connected  in  series  with '  the 
feeder  circuit,  is  added  to  the  gen- 
erator voltage,  thus  raising  the  feeder 
voltage.  For  the  other  position  of 
the  reversing  switch,  the  induced 
voltage  in  the  secondary  turns,  which 
are  in  series  with  the  feeder  circuit,  is  subtracted  from  the  gen- 
erator voltage,  thus  reducing  the  feeder  voltage. 

When  the  arm  of  the  dial  switch  touches  two  adjacent  contact 
points  (and  it  must  be  arranged  to  always  touch  one  point  before  it 
leaves  the  other),  the  intervening  turn  (or  turns)  of  the  secondary 
coil  of  the  regulator  is  short-circuited.  To  overcome  this  difficulty, 
the  arm  is  made  double,  that  is,  two  arms  A  and  B  move  together 
side  by  side  as  shown  in  Fig.  389.  These  arms  are  shown  connected 

to  two  contact  points,  say 
Id  and  16.  C  represents  a 
special  form  of  choke  coil 
consisting  of  two  windings 
on  one  iron  core.  These 
two  windings  are  arranged  so  that  equal  currents  flowing  out  from 
A  and  B  circulate  around  the  core  in  opposite  directions,  so  that  the 


Fig.   388.     Complete  Internal  Connec- 
tions for  Stillwell  Voltage  Regulator 


Fig.  389. 


Diagram  of  Special  Form  of  Dial  Switch 
for  Stillwell  Regulator 


446 


ALTERNATING-CURRENT  MACHINERY 


.Fig.  390.     Diagram  of  Magnetic 
Voltage  Regulator 


core  is  not  magnetized,  and  the  windings  of  C  have  no  choking  action. 
When,  however,  the  two  fingers  A  and  B  touch  adjacent  points  of  the 

dial  switch,  the  turns  of  wire  S  on  the  reg- 
ulator secondary  tend  to  send  a  very  large 
current  out  on  A,  say,  and  back  on  B. 
These  oppositely  flowing  currents  circu- 
late around  the  core  of  C  in  the  same  di- 
rection. These  currents,  therefore,  mag- 
netize the  core,  and  the  windings  have 
in  consequence  a  very  considerable  chok- 
ing action,  the  effect  being  to  choke  down 
oppositely  flowing  currents  in  the  fingers 
A  and  B,  and  to  allow  currents  in  the 
same  direction  to  flow  freely  through  it. 

(b)  Magnetic  Voltage  Regulator.  The  type  of  voltage  regulator 
mentioned  under  (b)  is  sometimes  called  the  magnetic  voltage 
regulator.  A  laminated  iron  ring  RRR.R,  Fig.  390,  has  four  large 
deep  slots  on  its  inner  face  in  which  the  primary  coil  P  P  and  the 
secondary  coil  S  S  are  placed.  A  laminated  core  C  C  mounted  on  a 
spindle  is  arranged  to  be  turned  into  any  desired  position  by  means 

of  a  hand  wheel.  In  the  posi- 
tion indicated  by  full  lines,  the 
core  carries  the  magnetic  flux 
due  to  the  primary  coil  in  one 
direction  through  the  second- 
ary coil,  and  in  the  position 
indicated  by  dotted  lines,  the 
core  carries  the  magnetic  flux 
due  to  the  primary  coil  in  the 
other  direction  through  the 
secondary  coil.  Therefore, 
when  the  core  is  moved  slowly 
from  position  1  to  position  2, 
in  the  direction  indicated  by 
the  arrows,  the  voltage  in- 
duced by  the  secondary  coil  changes  gradually  from  a  full  positive 
value  to  an  equal  negative  value  in  its  relation  to  the  primary  volt- 
age. That  is,  when  the  core  is  in  position  1,  the  induced  voltage  in  the 


Fig.  391.     Magnetic  Voltage  Regulator  in  Case 
with_Cover  Removed 


ALTERNATING-CURRENT  MACHINERY 


447 


secondary  coil  has  its  greatest  value  of  say,  100  volts  which,  if  the 
coils  are  properly  connected,  is  added  to  the  bus-bar  voltage  E,  giving 
a  feeder  voltage  of  E-f- 100.  When  the  core  is  midway  between  the  two 
positions  1  and  2,  the  induced  voltage  in  the  secondary  coil  is  zero, 
and  the  feeder  voltage  is  then  equal  to  the  bus-bar  voltage  E.  When 
the  core  is  in  position  2,  the  induced  voltage  in  the  secondary  coiLis 


Fig.  392.     General  Electric  Six-Phase  Induction^ 
Regulator  Complete  in  Case 

again  at  a  maximum  of  say,  100  volts,  but  in  such  a  direction  as  to 
oppose  the  bus-bar  voltage,  so  that  the  feeder  voltage  is  E— 100  volts. 
Fig.  391  is  a  view  of  a  magnetic  voltage  regulator  with  the 
covering  of  its  containing  case  removed.  The  two  coils,  primary  and 
secondary,  at  right  angles  to  each  other,  are  clearly  shown  with 
their  leads  passing  out  to  the  connection  board  which  occupies  the 
compartment  on  the  back  of  the  case  in  the  figure.  The  hand  wheel 


448  ALTERNATING-CURRENT  MACHINERY 

for  turning*the;iron  core  is  also  shown,  A  valuable  feature  of!  this 
type  of  regulator  is  that  it  produces  a  continuous  variation  of  vol- 
tage, whereas  the  Stillwell  regulator  produces  a  step-by-step  variation, 
(c)  Induction  Regulator.  The  combination  polyphase  induc- 
tion regulator  is  called  the  induction  regulator  from  its  similarity 
to  the  induction  motor.  The  action  of  the  induction  regulator 
stated  in  simplest  terms  is  as  follows:  A  regular  induction  motor 
stator  has  its  polyphase  windings  connected  across  the  polyphase 
bus  bars.  This  produces  a  rotating  magnetism  in  the  stator  iron  as 
explained  on  page  372.  This  rotating  magnetism  rotates  in  synchron- 
ism with  the  generator  or  generators  which  are  supplying  current  to 
the  bus  bars.  Inside  of  this  induction  motor  stator  (or  primary)  is 
placed  a  polyphase  armature  which  does  not  revolve,  but  it  is  mounted 


Fig.  393.     Laminated  Core  of  the  Stationary  Member 
^of  an  Induction  Regulator 

on  a  spindle  so  that  it  may  be  turned  through  an  angle  of  60°  or  90°  by 
means  of  a  hand  wheel  and  worm  gear.  The  rotating  stator  mag- 
netism induces  polyphase  electromotive  forces  in  the  windings  of 
this  polyphase  armature;  these  polyphase  electromotive  forces  are 
in  synchronism  with  the  electromotive  forces  between  the  bus  bars, 
and  the  two  (or  more)  windings  of  the  polyphase  armature  are  con- 
nected in  series  with  the  two  (or  more)  feeder  circuits  (constituting 
of  course  one  set  of  polyphase  feeders)  which  are  to  be  regulated. 
The  electromotive  forces  in  the  stationary  armature  windings  may 
be  in  phase  with  the  bus-bar  electromotive  forces,  in  which  case  the 
regulator  raises  the  voltage  by  the  greatest  amount  of  which  it  is 


ALTERNATING-CURRENT  MACHINERY 


449 


capable.  By  turning  the  stationary  armature  by  means  of  the  hand 
wheel  and  worm  gear,  the  phase  difference  between  the  bus-bar 
voltages  and  the  voltages  induced  in  the  stationary  armature  windings 
may  be  gradually  changed  from  coincidence  of  phase  to  opposition 
of  phase,  during  which  time  the  boosting  effect  of  the  regulator 
will  gradually  drop  to  zero,  become  negative,  and  reach  its  greatest 
negative  value  when  opposition  of  phase  is  reached.  Thus,  if  the 
•electromotive  force  induced  in  each  armature  winding  of  the  regulator 
is  100  volts,  and  if  the  bus-bar  voltage  is 
1,000  (each  phase),  then  the  voltage  between 
the  feeders  can  be  varied  from  900  volts 
to  1,100  volts  by  means  of  the  regulator. ' 

Fig.  392  is  a  general  view  of  a  six-phase 
induction  regulator  in  its  containing  case, 
manufactured  by  the  General  Electric  Com- 
pany. This  particular  machine  has  the 
primary  windings  on  the  movable  member, 
whereas  the  armature  windings  are  on  the 
stationary  member.  It  is  used  for  control- 
ling the  alternating-current  voltages  applied 
to  the  collector  rings  of  a  six-phase  rotary 
converter.  The  figure  shows  a  small  direct- 
current  motor  mounted  on  the  regulator 
case  for  turning  the  movable  member  of 
the  regulator,  this  motor  being  controlled  by 
the  hand  wheel  shown  in  the  figure. 

Fig.  393  shows  the  laminated  iron  core  of  the  stationary  mem- 
ber of  an  induction  regulator,  the  stampings  of  which  are  clamped 
in  a  cast-iron  shell.  Fig.  394  shows  the  movable  core. 

Ratings  of  Voltage  Regulators.  It  was  shown,  page,  253  that  the 
total  amount  of  power  delivered  to  service  mains  at  a  slightly  in- 
creased voltage  produced  by  an  autotransformer  is  very  much 
greater  than  the  power  actually  transformed  from  the  primary  to 
the  secondary  of  the  transformer.  In  fact,  the  power  actually 
transformed  is  equal  to  the  increase  (or  decrease)  of  voltage  mul- 
tiplied by  the  total  current  delivered,  and  the  rating  of  the  auto- 
transformer  (which  determines  its  size)  is  based  upon  the  power 
transformed. 


Fig.  394.     Movable  Core  of 

the  Primary  Member  of  an 

Induction  Regulator 


450  ALTERNATING-CURRENT  MACHINERY 


For  example,  a  voltage  regulator  is  to  be  used  for  raising  the 
voltage  of  2,000- volt  bus  bars  to  a  maximum  of  2,100  volts,  and  the 

XR 


Fig.  395.     Diagram  of  Connections  for  Voltmeter 
Compensator 

maximum  current  to  be  handled  is  100  amperes.  In  this  case  the 
transformer  rating  of  the  regulator  is  100  amperes  at  100  volts  (or 
10  kilowatts),  whereas  the  total  power  to  be  delivered  to  the  feeders 
is,  at  its  maximum,  2,100  voltsXIOO  amperes  or  210  kilowatts. 

Since  a  voltage  regulator  transforms  only  a  small  fraction  of 
the  power  delivered  to  the  feeders  which  it  controls,  the  losses  of 
power  in  the  regulator  are  very  small  indeed.  Thus  the  10-kilowatt 
regulator  might  have  a  total  loss  of  300  watts,  which  is  3  per  cent 
of  the  power  actually  transformed  by  the  regulator,  and  only  one- 
seventh  of  one  per  cent  of  the  total  power  delivered  to  the  feeders. 

Voltmeter  Compensator.  The  voltmeter  compensator  is  a  device 
by  means  of  which  the  voltmeter  on  the  switchboard  in  a  station  is 
made  to  indicate  the  voltage  between  the  transmission  lines  at  some 
remote  feeding  center  or  receiving  station.  The  essential  principles 
of  this  instrument  are  made  clear  from  Fig.  395.  An  alternating- 
current  generator  G  delivers  alternating  current  at  voltage  EQ  be- 


Figs.  396  and  397.      Vector  Diagrams   Showing  E.M.F. 
Relations  in  the  Line  and  in  the  Compensator 

tween  its  terminals.    This  current  is  transmitted  over  a  line  of  which 
the  resistance  is  R  (including  r)  and  the  reactance  is  X  (including 


ALTERNATING-CURRENT  MACHINERY 


451 


x),  and  the  voltage  at  the  receiver  is  E.    The  line  loss  of  electro- 
motive force  is  XI  due  to  reactance,  and  IR  due  to  resistance,  as 


XR 


Fig.  398.     Wiring  Diagram  for  Voltmeter  Compensator  as 
Used  in  Practice  in  High  Voltage  Circuits 

explained  on  page  58.  The  relation  between  E0,  E,  IR,  and  XI  \s 
shown  in  Fig.  396.  T,  Fig.  395,  is  a  transformer  which  supplies  to 
the  voltmeter  V  a  voltage  exactly  in  phase  with  E0,  and,  say,  one- 


Fig.  399.     Diagram  of  Circuits  and  Apparatus  in 
Connection  with  Westinghouse  Compen- 
sated Voltmeter 

tenth  as  great.     Let  x  be  a  reactance  one-tenth  as  great  as  X,  and 
r  a  resistance  one-tenth  as  great  as  R.    Then  the  voltage  between 


452  ALTERNATING-CURRENT  MACHINERY 

the  points  a  and  b  consists  of  two  parts,  rl  and  xl,  which  are  in  phase 
with,  and  one-tenth  as  great  as,  IR  and  X  /,  respectively.  There- 
fore rl  and  xl  subtracted  from  one-tenth  E0  give  a  voltage  which  is 
exactly  equal  to,  and  in  phase  with,  one-tenth  E,  as  shown  in  Fig. 

397,  so  that  the  voltmeter  being  acted  upon  by  (one-tenth  E-^  rl—  xl) 
gives  a  reading  which  multiplied  by  10  is  equal  to  E. 

In  practice,  it  is  not  desirable  to  connect  the  voltmeter  wires 
to  the  high-voltage  mains,  and  the  essential  features  of  the  arrange- 
ment shown  in  Fig.  395  are  realized  by  introducing  r  and  x  in  series 
with  the  secondary  of  a  current  transformer  T7',  as  shown  in  Fig. 

398.  More  or  less  of  the  resistance  r  and  of  the  reactance  x  may  be 
included  in  the  voltmeter  circuit  by  means  of  two  dial  switches  of 
which  the  arms  are  represented  by  d  d,  Fig.  398.    The  object  of  these 
dial  switches  is  to  enable  a  given  compensator  to  be  adjusted  to  any 
given  transmission  line. 

Fig.  399  shows  all  of  the  apparatus  used  in  connection  with 
the  Westinghouse  compensated  voltmeter;  the  lines  II  are  shown 
at  the  left,  T  is  the  step-down  potential  transformer,  T'  is  the  series 
transformer,  dd  is  the  case  containing  the  resistance  r  and  the 
reactance  x,  and  upon  which  the  dial  switches  are  mounted,  V  is 
the  voltmeter,  -and  VR  is  the  resistance  in  series  with  the  voltmeter. 

LIGHTNING  ARRESTERS 

Effects  of  Lightning.  When  a  lightning  stroke  occurs  in  the 
neighborhood  of  a  transmission  line,  a  sudden  rush  of  electric  cur- 
rent takes  place  over  the  line  due  to  one  or  more  of  the  following 
causes : 

(a)  Electric  charge  accumulated  on  the  line  is  suddenly  released  and 
tends  to  flow  to  earth. 

(b)  The  magnetic  action  of  the  lightning  discharge  induces  a  sudden 
rush  of  current  in  the  line. 

(c)  When  the  lightning  discharge  actually  strikes  the  line,  an  enor- 
mous rush  of  current  takes  place  over  the  line  and  to  earth. 

When  the  sudden  rush  of  current  which. accompanies  a  light- 
ning discharge  encounters  a  portion  of  the  circuit  which  has  con- 
siderable inductance,  very  great,  electromotive  forces  are  created, 
in  the  same  way  that  enormous  mechanical  forces  are  created  when 
a  moving  body  strikes  a  heavy  obstacle,  and  the  insulation  of  the 
circuit,  be  it  air  or  solid  insulation,  is  likely  to  be  broken  down,  or 


ALTERNATING-CURRENT  MACHINERY 


453 


punctured,  giving  a  short  path  to  the  earth  for  the  rush  of  current. 
Thus,  a  sudden  rush  of  current  coming  into  a  station  over  a  trans- 
mission line,  encounters  the  highly  inductive  windings  of  wire  of 
a  transformer,  dynamo,  or  other  apparatus.  The  electrical  inertia 
(inductance)  of  the  windings  dams  up  the  rush  of  current,  as  it 
were,  and  the  current  rush  is  almost  sure  to  break  through  the 
insulation  at  the  very  entrance  to  the  winding,  passing  from  the 
copper  wire  over  to  any  metal  which  is  connected  more  or  less 
thoroughly  to  earth,  resulting  in  the  coils  or  apparatus  being  burned 
out  or  badly  damaged. 

A  lightning  arrester  is  a  device  for  shielding  a  transformer, 
dynamo,  or  other  piece  of  apparatus  from  the  rushes  of  current 
which  come  into  a  station  on  an  overhead  line  during  a  thunder- 
storm and  from  disturbances  due  to  the  static  unbalancing  of  the 
electrical  circuits. 

The  lightning  arrester  consists  of: 

(a)  An    inductance 
coil,   or  choke  coil  as  it  is 
called,  for  damming  up  more 
or  less  completely  the  rush 
of  current  before  it  reaches 
the  dynamo  or  other  appa- 
ratus. 

(b)  A    weak    place, 
specially  arranged  in  the  in- 
sulation of  the  line,  that  is, 
a  spark  gap,  just  in  front  of 
the  choke  coil  through  which 
the  dammed  up  rush  of  cur- 
rent may   break,  and   flow 
harmlessly  to  earth. 

(c)  A  conducting  path  to  earth  as  straight  and  direct  as  possible,  and 
a  good  earth  connection.     Turns  and  bends  in  the  conducting  wire  connected 
to  earth  are  to  be  avoided,  inasmuch  as  they  introduce  inductance  which 
tends  to  check  the  quick  flow  of  current  to  earth. 

(d)  A  device  for  extinguishing  the  electric  arc  which  may  be  main- 
tained across  the  spark  gap  by  the  regular  line  current  itself,  after  the  rush 
of  current  from  the  lightning  discharge  has  passed  and  gone. 

The  choke  coil  of  a  lightning  arrester  must  be  very  highly  in- 
sulated. Two  types  of  choke  coil  are  commonly  used.  Fig.  400 
shows  a  cylindrical  choke  coil  mounted  on  a  marble  base,  and  con- 
sisting of  three  layers  of  large  and  very  highly  insulated  wire.  The 


Fig.  400.     Choke  Coil  Mounted  on  Marble  Base 


454  ALTERNATING-CURRENT  MACHINERY 


terminals  of  the  coil  are  at  its  two  ends.     Fig.  401  shows  a  choke 
coil  consisting  of  an  insulated  copper  strip  wound  like  a  roll  of  tape. 

One  terminal  of  this  choke  coil  is  on  the 
outer  edge  and  the  other  is  at  the  center. 
The  figure  shows  the  method  of  supporting 
the  coil  on  a  special  insulator  carried  on  a 
bracket.  Iron  cores  do  not  add  to  the 
damming  action  of  a  choke  coil  in  the  case 
of  the  excessively  quick  rushes  of  current 
which  accompany  lightning  discharges,  for 
the  reason  that  the  iron  does  not  have 
time  to  become  magnetized.  Therefore, 
lightning  arrester  choke  coils  are  always 
made  without  iron  cores. 

Multi=Gap  Non=Arcing  Arrester.    The 
spark  gap  of  alternating-current  lightning 
arresters  is  usually  made  between  blocks 
of  metal  containing  zinc  or  cadmium.    A. 
J.  Wurtz  made  the  discovery  that  it  takes 
a  very  high  voltage  to  maintain  an  alter- 
nating-current arc   between  metal  elec- 
trodes which  contain  zinc  or  cadmium.     The  behavior  of  these  alloys 
is  somewhat  analogous  to  that  of  the  mercury-vapor  rectifier  described 

on  page  321  in  that  reversal  of  the  cur- 
rent flow  which  makes  the  mercury  an 
anode  requires  a  greatly  increased  volt- 
age. If  this  is  not  supplied  the  flow 
could  not  be  maintained.  Thus  an  al- 
ternating electromotive  force  of  a  fre- 
quency of  60  cycles  per  second,  and  500 
volts  (effective  value),  cannot  main- 
tain an  arc  across  a  &-inch  air  gap 
between  massive  blocks  of  brass.  This 
non^arcing  property  of  certain  alloys  is 
made  use  of  in  the  multi-gap  arrester  of 
Fig.  402.  westinghouse  Multi-Gap  the  Westinghouse  Electric  Company. 

Lightning  Arrester  .          . 

shown  in  Fig.  402.    It  consists  of  seven 
independent  knurled  brass  cylinders  supported  by  two  overhanging 


Fig.  401.     Choke  Coil  made  of  In 
sulated  Copper  Strip 


ALERNATING-CURRENT  MACHINERY 


455 


porcelain  blocks  forming  a  unit  which  is  mounted  in  a  weatherproof 
cast-iron  case.  The  two  end  cylinders  are  connected,  respectively,  to 
the  two  line  wires  and  the  center  cylinder  is  connected  to  ground.  This 
arrester  may  also  be  used  single  pole,  one  for  each  side  of  a  two-wire  cir- 
cuit. Connections  for  single-phase  and  for  three-phase  circuits  are 
given  in  Fig.  403.  For  two-phase  circuits  each  phase  is  to  be  con- 
nected as  in  the  single-phase  diagram. 

This  type  of  arrester  is  suited  for  use  within  a  radius  of  three 
miles  from  the  source  of  power  on  systems  of  200-kw.  capacity  or 
less,  and  up  to  400  kilowatts  when  connected  more  than  three  miles 
away  from  the  power  source. 


•5/rtGLE  PHASE 
CESS    THAM  12  50 -VOLTS 


S/MGL  E  PHASE 
1Z5O   TO  25OO.  VOLTS 


THREE    PHASE 

THAM  /25O  VOLTS 


THREE    PHASE 
1250  TO   2500  VOLTS 


Fig.  403.     Lightning  Arrester  Connections  for  Single-Phase  and  Three- Phase  Circuits 

This  type  of  arrester  is  not  recommended  for  protecting  circuits 
liable  to  low  power  factors,  nor  for  25-cycle  circuits,  for  experience 
has  shown  that  under  these  conditions  the  arcing  cannot  be  suppressed. 

Multi=Path  Arrester.  The  multi-path  arrester  of, .the  Westing- 
house  Company  is  single  pole  and  adapted  to  either  alternating-  or 
direct-current  circuits  of  any  voltage  up  to  1,000.  It  can  be  mounted 
upon  a  pole.  It  consists  of  a  spark  gap  in  series  with  a  high  resist- 
ance block  of  carborundum.  This  block  offers  a  very  high  resistance 
under  normal  conditions  and  moderate  voltages,  but  its  resistance 
to  a  static  discharge  is  very  low,  and  it  readily  conducts  a  lightning 


456  ALTERNATING-CURRENT  MACHINERY 

discharge  to  ground.  After  the  discharge  has  passed,  it  resumes  its 
normal  high  resistance,  thus  preventing  the  line  voltage  from  main- 
taining an  arc.  Fig.  404  is  a  view  of  the  arrester  in  the  lower  half 
of  its  iron  casing,  the  upper  half  being  shown  removed. 

Electrolytic=Cell  Lightning  Arrester.  This  type  of  arrester, 
which  has  proved  highly  effective,  is  widely  used.  A  single  cell  con- 
sists of  two  aluminum  plates  and  an  electrolyte,  such  as  ammonium 
phosphate,  which  form  a  condenser  that  will  stand  about  340  volts 
alternating  before  breaking  down.  Up  to  this  critical  value  of  the 
voltage,  which  varies  somewhat  with  the  electrolyte  used,  a  very  small 
current  passes  through  the  cell,  but  as  soon  as  the  critical  voltage 


Fig.  404.     Westinghouse  Multi-Path  Lightning  Arrester 
— Upper  Half  of  Case  Removed 

is  exceeded,  the  current  increases  very  rapidly  with  even  a  slight 
increase  in  the  voltage.  As  soon,  however,  as  the  voltage  drops 
below  the  critical  value,  the  resistance  of  the  cell  increases  greatly. 

In  applying  this  interesting  property  of  the  aluminum  cell  to 
the  lightning  arrester,  a  large  number  of  cells  are  made  up  in  series 
and  mounted  in  standard  size  units  to  be  connected  in  series,  the 
number  of  units  chosen  depending  upon  the  voltage  of  the  lines  to 
be  protected,  allowing  on  the  average  about  300  volts  per  cell. 

Fig.  405  shows  a  cross-section  of  an  aluminum  cell  lightning 
arrester  as  made  by  the  General  Electric  Company.  It  consists  of 


ALTERNATING-CURRENT  MACHINERY 


457 


TO  HORN  CAP 


a  series  of  concentric  inverted  cones  of  aluminum  placed  one  above 
the  other  with  a  vertical  spacing  of  about  0.3  inch.  The  electrolyte 
is  poured  into  the  cones  and  partly  fills  the  space  between  adjacent 
ones.  The  pile  of  cones  with  the  electrolyte  between  them  is  then 
immersed  in  a  tank  of  oil,  which  helps  to  insulate  cones  from  each 
other  except  for  the  electrolyte,  and  also  prevents  evaporation  of 
the  solution.  A  cylinder  of  insulating  fiber  concentric  with  the  stack 
of  cones  is  placed  between  the  latter  and  the  steel  tank.  This  arrange- 
ment assists  the  free  circulation  of  the  oil  and  increases  the  insulation 
between  tank  and  aluminum  cones. 

A  stack  of  cones  is  used  for  each 
phase,  and  up  to  7,250  volts  all  the  stacks 
are  placed  in  a  single  tank.  Where  the 
line  voltage  exceeds  7,250,  each  stack  of 
cones  is  placed  in  a  separate  tank.  The 
connection  of  the  stacks  to  the  line  wires 
through  spark  gaps  and  to  the  ground  is 
different,  depending  upon  whether  the 
circuits  have  their  neutral  point  purposely 
grounded  or  not,  and  upon  the  line  volt- 
age. 

The  top  cone  of  each  stack  is  not 
directly  connected  between  a  line  wire 
and  the  ground,  because  at  normal  line 
voltage  some  current  would  flow  contin-  Fig.  405.  Cross-Section  of  Aiumi- 

.       .  .       .         .         .  „          ,.   ,  num  Cell  Lightning  Arrester 

uously  through  the  aluminum  cells,  which 

would  heat  them  and  evaporate  the  electrolyte.  To  prevent  this 
flow  of  current,  the  aluminum  stacks  are  in  practice  connected  to 
the  line  wires  through  spark  gaps  adjusted  for  a  sparking  voltage 
just  above  that  of  the  normal  line  voltage.  Under  normal  conditions 
no  current  passes  from  the  line  wires  through  the  aluminum  cells, 
but  a  lightning  discharge  with  its  accompanying  high  voltage  easily 
jumps  the  spark  gap  and  passes  through  the  aluminum  cells  to  ground. 
As  soon,  however,  as  the  discharge  is  over,  the  resistance  of  the 
spark  gap  in  series  with  the  aluminum  plates  is  too  large  to  permit 
the  line  voltage  to  maintain  an  arc. 

Fig.  406  is  a  view  showing  a  typical  installation  of  an  aluminum 
lightning  arrester  for  a  6,600-volt,  three-phase,  non-grounded  neutral 


458  ALTERNATING-CURRENT  MACHINERY 

circuit.  As  shown  the  three  stacks  of  cones,  one  for  each  phase, 
are  installed  in  one  tank  which  is  placed  on  a  rack  insulated  from 
the  ground.  Two  of  the  stacks  are  connected  directly  to  two  of  the 


Fig.  406.     Typical  Installation  for  Aluminum  Lightning 
Arrester  on  Three-Phase  Non-Grounded  Neutral  Circuit 

three  line  wires,  each  through  a  "horn-gap";  the  third  is  connected 
to  the  remaining  line  wire  through  a  "transfer  device"  or  rotating 
switch,  and  a  horn  gap.  The  fourth  stack  is  connected  to  the  ground 
through  the  transfer  device,  which  may  be  explained  as  follows : 

When  the  stacks  of  aluminum  cones  have  been  disconnected 
from  the  line  wires  for  a  certain  length  of  time,  the  insulating  film 


ALTERNATING-CURRENT  MACHINERY  459 

on  the  cones  deteriorates,  and  experience  has  shown  that  in  order 
to  keep  the  aluminum  cells  in  good  working  order,  they  must  be 
connected  to  the  line  wires  and  charged  at  certain  definite  intervals. 
In  some  cases  this  type  of  arrestor  must  be  charged  daily,  and  in 
others  weekly.  The  charging  process  consists  simply  of  short-circuit- 
ing the  spark  gaps  between  the  stacks  and  the  line  wires  for  a  few 
moments.  The  transfer  device  provides  a  means  of  interchanging 
the  ground  stack  with  one  of  the  line  stacks  during  the  charging 
operation  so  that  tne  insulating  films  of  all  the  cells  may  be  reformed 
to  an  equal  extent.  For  arresters  up  to  27,000  volts,  the  device  is 
mounted  with  three  insulators  on  the  pipe  frame  work  and  is  operated 
by  a  hand- wheel,  as  shown  in  Fig.  406. 

Combination  of  a  Condenser  with  a  Choke  Coil.  The  choking 
action  of  a  coil  of  wire  is  due  to  its  electrical  inertia  or  inductance, 

CHOKE  COIL 

TO  LINE         b        ///J^V?\\\ b      TO  APPARATUS 


GAPS 


III 


GROUND  GROUND 

Fig.  407.     Combination  of  Condenser  and  Choke 
Coil  as  Lightning  Arrester 

and  the  shielding  of  the  apparatus  behind  a  choke  coil  from  severe 
electrical  strains  is  precisely  analogous  to  the  following  mechanical 
arrangement : 

A  wall  is  shielded  from  the  severe  strains  due  to  a  hammer  blow  by 
allowing  the  hammer  to  strike  against  a  very  heavy  ball  of  iron  which  rests 
against  the  wall.  The  completeness  with  which  this  heavy  iron  ball  shields 
the  wall  depends  upon  the  interposing  of  an  elastic  substance  between  the 
ball  and  the  wall.  The  ball  alone  cannot  shield  the  wall  unless  the  wall  itself 
is  slightly  elastic. 

In  the  same  way  the  completeness  with  which  a  choke  coil 
shields  the  dynamo  and  other  apparatus  depends  upon  the  presence 
of  some  electro-elasticity*,  or  capacity  in  the  circuit  behind  the 
choke  coil.  In  some  cases  the  circuit  behind  the  choke  coil  has  suf- 


*See 


460  ALTERNATING-CURRENT  MACHINERY 

ficient  capacity  for  this  purpose;  it  is  always  best,  however,  to  con- 
nect a  condenser  behind  the  choke  coil  as  shown  in  Fig.  407. 

The  choke  coil  and  condenser  are  immersed  in  an  oil  tank 
when  installed.  From  Fig.  407  it  is  seen  that  three  wires  bbb  con- 
nect to  choke  coil  and  condenser.  This  combination  of  choke  coil 
and  condenser  is  called  a  static  interrupter  by  the  Westinghouse 
Company. 

INSTRUCTIONS  FOR  INSTALLING  LIGHTNING  ARRESTERS 

Location.  As  regards  the  location  of  lightning  arresters,  electric  plants 
may  be  divided  into  two  groups: 

(a)  Plants  in  which  the  individual  pieces  of  apparatus  such  as  trans- 
formers, motors,  arc  lights,  etc.,  are  many  in  number  and  widely  scattered. 
In  these  cases  lightning  arresters  should  be    ocated  for  the  purpose  of  pro- 
tecting the  whole  line.     They  should  be  located  at  a  number  of  points,  more 
numerous  on  the  parts  of  the  line  particularly  exposed,  and  fewer  in  number 
on  the  parts  that  are  naturally  protected,  especially  those  parts  shielded  by 
tall  buildings  or  numerous  trees.     No  definite  statement  can  be  made  as  to 
the  number  of  arresters  needed  per  mile,  as  the  requirements  of  different  cases 
vary  widely.     Under  average  conditions  no  point  of  the  circuit  should  be 
more  than  1,000  feet  from  an  arrester.     It  is  not  usual  to  find  distributed 
apparatus  on  circuits  of  over  2,500  volts,  but  when  such  cases  occur,  a  light- 
ning arrester  should  be  placed  as  near  as  possible  to  each  piece  of  apparatus. 

(b)  Plants  in  which  the  apparatus  is  located  at  a  few  definite  points 
in  the  system,  as  in  a  high-voltage  transmission  line.     In  such  cases  the  ar- 
resters should  in  general  be  lecated  to  protect  especially  those  points  where 
apparatus  is  situated,  that  is,  should  be  placed  with  the  object  of  protecting 
the  apparatus  rather  than  the  line  as  a  whole. 

The  lightning  arrester  should  always  be  so  connected  that  in  passing 
from  the  line  to  the  apparatus  the  arrester  is  reached  first,  the  choke  coil 
second,  and  the  condenser,  if  one  is  used,  third. 

Insulation.  A  lightning  arrester  is  naturally  exposed  to  severe  volt- 
age strains  and,  therefore,  all  active  parts  must  be  well  insulated.  On  ar- 
resters for  low  voltages  it  is  not  a  difficult  matter  to  secure  proper  insulation, 
as  the  construction  of  the  arrester  itself  affords  protection.  On  high-voltage 
arresters,  however,  proper  insulation  is  a  more  difficult  matter.  All  arresters 
are  marble  or  porcelain  mounted,  the  marble  or  porcelain  serving  as  an  insu- 
lating support  for  the  arrester.  On  circuits  exceeding  6,000  volts,  to  obtain 
further  insulation,  the  marble  or  porcelain  bases  should  be  mounted  on  wooden 
supports,  well  dried  and  shellaced.  For  12,000  volts  and  above,  the  bases 
or  panels  should  receive  additional  insulation  in  the  form  of  porcelain  or 
glass  insulators  used  as  supports. 

Two  high-voltage  arresters  attached  to  different  line  wires  should  not 
be  placed  side  by  side  without  either  a  barrier  or  a  considerable  space  between 
them. 

Grounds.  Too  much  importance  cannot  be  attached  to  the  making 
of  proper  ground  connections  which  should  be  as  short  and  straight  as  pos- 


ALTERNATING-CURRENT  MACHINERY  461 

sible.  A  poor  ground  connection  will  render  ineffective  every  effort  made 
with  choke  coils  and  lightning  arresters  to  divert  static  electricity  into  the 
earth.  It  is  important,  therefore,  to  not  only  construct  a  good  ground  con- 
nection, but  to  maintain  it  so. 

A  good  ground  connection  for  a  bank  of  station  arresters  may  be  made 
as  recommended  by  the  Westinghouse  Company  in  the  following  manner: 
First,  dig  a  hole  4  feet  square  as  near  the  arrester  as  possible  until  perma- 
nently damp  earth  has  been  reached;  second,  cover  the  bottom  of  this  hole 
with  crushed  charcoal  (about  pea  size);  third,  over  this  lay  10  square  feet  of 
tinned  copper  plate;  fourth,  solder  the  ground  wire,  preferably  No.  0  copper, 
securely  across  the  entire  surface  of  the  ground  plate;  fifth,  cover  the  ground 
plate  with  crushed  charcoal;  and  sixth,  fill  the  hole  with  earth,  using  running 
water  to  settle. 

The  above  method  of  making  a  ground  connection  is  simple,  and  has 
been  found  to  give  excellent  results,  and  yet,  if  not  made  in  proper  soil,  it 
will  prove  of  little  value.  Where  a  mountain  stream  is  conveniently  near 
it  is  not  uncommon  to  throw  the  ground  plate  into  the  bed  of  the  stream. 
This,  however,  makes  a  poor  ground  connection,  owing  to  the  high  resistance 
of  the  pure  water  and  the  rocky  bottom  of  the  stream.  Clay,  even  when 
wet,  rock,  sand,  gravel,  dry  earth,  and  pure  water  are  not  suitable  materials 
in  which  to  bury  the  ground  plate  of  a  bank  of  lightning  arresters.  Rich 
soil  is  the  best.  It  is,  therefore,  advisable,  before  installing  a  bank  of  choke 
coils  and  lightning  arresters,  to  select  the  best  possible  site  for  the  lightning 
arrester  installation  with  reference  to  a  good  ground  connection.  This  may 
often  be  at  some  little  distance  from  the  station,  in  which  case  it  is,  of  course, 
necessary  to  construct  a  lightning-arrester  house.  Where  permanent  damp- 
ness cannot  be  reached  it  is  recommended  that  water  be  supplied  to  the  ground 
through  a  pipe  from  some  convenient  source. 

Where  possible,  a  direct  connection  to  an  underground  pipe  system, 
especially  of  a  town  or  city  water  main,  furnishes  an  exce]Jent  ground  on  ac- 
count of  the  great  surface  of  pipe  in  contact  with  the  earth  and  the  numer- 
ous alternative  paths  for  the  discharge.  In  a  water-power  plant  the  ground 
should  always  include  a  connection  to  the  pipe  line  or  penstock  and  to  the 
case  or  frame  of  the  apparatus  to  be  protected.  An  effective  and  easily  made 
ground  may  be  effected  by  using  a  large,  old  iron  casting,  like  an  old  car 
wheel,  fitted  with  a  riveted  copper  strap  and  buried  in  damp  earth.  A  few 
pounds  of  common  salt  thrown  around  any  ground  terminal  before  covering 
helps  to  maintain  dampness. 

Inspection.  As  the  effectiveness  of  every  arrester  is  of  great  impor- 
tance, they  should  be  inspected  from  time  to  time  and  the  resistances  and 
earth  connection  tested  for  open  circuit. 


462  ALTERNATING-CURRENT  MACHINERY 

APPENDIX 

PARALLEL  OPERATING  OF  ALTERNATORS 

Necessary  Conditions.  In  the  parallel  or  multiple  operation 
of  alternators,  it  is  necessary  that  they  be  similar  in  three  respects 
in  order  to  Insure  their  working  together  properly  when  connected 
in  parallel,*  viz,  frequency,  phase,  and  voltage. 

(a)  Frequency.     Two  generators  are  of  the  same  frequency 
when  the  numbers  of  alternations  or  reversals   of   their   electro- 
motive forces  in  a  given  time  are  equal.    This  requirement  is  fulfilled 
when  the  product  of  the  number  of  poles  by  the  revolutions  per 
minute  is  the  same  for  each  machine.    The  frequency  of  a  generator, 
being  dependent  upon  its  speed,  may  be  controlled  by  the  regulation 
of  the  speed  of  its  prime  mover. 

(b)  Phase.    Two  generators  are  in  phase  when  the  positions 
of  their  armatures  with  respect  to  their  field  poles  are  the  same, 
i.e.,  when  similar  armature  coils  are  opposite  positive  field  poles 
at  the  same  instants.     When  this  condition  exists  the  electromo- 
tive forces  of  the  machines  are  both  positive  at  the  same  time,  and 
their  maximum  values  occur  at  the  same  instant;  the  electromotive 
forces  are  said  to  be  coincident  in  phase. 

(c)  Voltage.    Two  generators  are  of  the  same  voltage  when 
the  pressure  me*asured  across  the  armature  terminals  is  the  same 
for  each  machine.    The  voltage  for  a  given  speed  being  dependent 
upon  the  field  strength  may  be  controlled  by  means  of  the  rheostat 
in  the  field  circuit. 

Determination  of  Relative  Frequency  and  Phase  Coincidence. 
If  two  similar  generators  are  running  at  exactly  the  same  speed  their 
difference  in  phase  remains  constant.  This  condition,  however, 
does  not  exist  in  practice  unless  the  armatures  are  rigidly  connected, 
as  the  inevitable  fluctuation  in  engine  and  water-wheel  speeds 
and  in  belt  slippage  causes  the  position  of  the  armatures  with  refer- 
ence to  their  field  poles  to  be  continually  changing,  and  consequently 
the  difference  between  the  phases  to  be  likewise  changing.  As 
generators  should  not  be  thrown  in  parallel  excepting  when  their 
frequencies  are  practically  the  same,  and  at  the  time  their  phases 

,     *Furthermore,  two  alternators,  to  operate  satisfactorily  in  parallel,  must  have  electro- 
motive force  waves  of  the  same  shape. 


ALTERNATING-CURRENT  MACHINERY 


463 


are  in  exact  coincidence,  or  nearly  so,  it  is  essential  to  have  an  accurate 
means  of  determining  when  these  conditions  exist. 

The  principle  of  the  most  common  method  of  determining 
when  generators  are  of  the  same  frequency  and  are  coincident  in 
phase,  is  illustrated  in  Fig.  408.  A  and  B  represent  two  single- 
phase  generators,  the  leads  of  which  are  connected  at  the  switch^C,- 
through  two  series  of  incandescent  lamps  D  and  E.  It  is  evident 
that  as  the  relative  positions  of  the  phases  of  the  electromotive 
forces  change  from  that  of  exact  coincidence  to  that  of  exact  oppo- 
sition, the  flow  of  current  through  the  lamps  varies  from  a  mini- 
mum when  the  machines  oppose  each  other  in  forcing  current  through 
the  lamps  as  shown  by  the  arrows  in  Fig.  408,  to  a  maximum  when 
the  machines  help  each  other  in  forcing  current  through  the  lamps.* 
If  the  electromotive  forces  of  the  two  machines  are  exactly  equal 
and  in  phase,  the  current  through  the  lamps  will  be  zero,  and  as  the 
difference  in  phase  increases,  the  lamps  will  light  up  and  will  in- 
crease in  brilliancy  until  the  maximum  is  reached;  when  the  phases 
are  in  exact  opposition.  From  this  condition  they  will  decrease  in 
brilliancy  until  completely  dark,  indicating  that  the  machines  are 
again  in  phase.  The  rate  of  pulsations  of  the  lamps  depends  upon 
the  difference  in  frequency,  i.e.,  upon  the  difference  in  the  speeds 
of  the  machines,  and  by  ad- 
justment of  the  governors  of 
the  engine  or  water-wheel,  or 
the  tension  of  the  belt,  the 
rate  can  generally  be  reduced 
to  as  low  as  one  pulsation 
in  ten  seconds,  which  affords 
ample  time  for  throwing  the 
switch  connecting  the  gen- 
erators in  parallel. 

Synchronizer.  When  the  phases  of  two  generators  coincide, 
the  machines  are  said  to  be  "in  phase,"  "in  step,"  or  "in  synchron- 
ism". The  apparatus  used  for  determining  when  generators  are  in 
phase  is  called  a  synchronizer.  In  Fig.  408  the  lamps  constitute  the 
synchronizer.  While  a  series  of  lamps,  alone,  may  be  used  for  syn- 


Fig.  408.     Wiring    Diagram    Showing    Method    of 

Determining  Coincidence  in   Phase  and 

Frequency  of  Two  Generators 


*When  the  two  machines  oppose  each  other  in  forcing  current  through  the  lamps  they 
would  help  each  other,  or  be  in  phase  with  each  other,  in  producing  current  in  the  receiving 
circuit  to  which  the  machines  are  to  be  connected  in  parallel. 


464 


ALTERNATING-CURRENT  MACHINERY 


chronizing  machines  of  very  low  voltage,  it  is  not  safe  or  practical 
to  use  this  method  for  machines  of  high  voltage.  The  most  common 
arrangement  for  synchronizing  alternators,  in  general,  is  illustrated 
in  the  diagram,  Fig.  409.  A  and  B  represent  two  single-phase 
generators  with  switches  in  the  main  leads.  There  are  two  trans- 
formers, the  primaries  of  which  are  connected  across  the  main  leads 
of  A  and  E,  respectively,  the  secondaries  being  connected  in  series 
through  the  lamps  E.  Now,  if  the  transformers  are  connected 
similarly  in  the  two  circuits,  as  shown  in  the  diagram,  then  when 
the  generators  A  and  B  are  in  phase,  the  electromotive  forces  in 
the  secondaries  will  be  in  phase  and  no  current  will  flow  through 
the  lamps,  but  when  the  generators  are  out  of  phase,  the  electro- 
motive forces  in  the  secondary  circuits  will  be  out  of  phase  also, 
and  current  will  flow  through  the  lamps.  The  amount  of  this  cur- 
rent and  the  resultant  brilliancy  of  the  lamps  depends  on  the  dif- 
ference in  phase.  If  the  connections  of  either  the  primary  or  the 

secondary  of  either  transformer 
i  be    now    reversed    from    those 


-o-o- 


Trans. 
A 


7>«/wi$3QQJ 

B 


shown  in  Fig.  409,  the  indications 
of  the  lamps  will  be  reversed; 
that  is,  when  the  generators  are 
in  phase  the  lamps  will  burn  at 
maximum  brilliancy,  and  vice 
versa.  It  is  the  common  prac- 
^ce  °^  the  Westinghouse  Com- 
pany to  arrange  the  transformer 
connections  so  that  the  lamps 
shall  be  dark  when  the  generators 
are  in  phase. 

In  order  to  determine  wheth- 
er the  synchronizer  lamps  will 
be  briSht  or  dark  for  a  given  con- 
nection of  transformers  when  the 
generators  are  coincident  in  phase,  remove  the  main  fuses  or  raise 
collector  brushes  from  one  machine,  and  throw  in  the  main  switches 
with  the  other  generator  at  full  voltage.  Since  both  primaries  are 
low  connected  through  the  switches  to  one  machine,  the  lamps  will 
be  in  the  same  condition  as  when  the  main  or  paralleling  switches 


bTth°d 


ALTERNATING-CURRENT  MACHINERY  465 

are  open  and  both  generators  are  coincident  in  phase.  If  the  lamps 
burn  brightly  and  it  is  desired  that  they  be  dark  for  an  indication 
of  phase  coincidence,  the  connections  of  one  of  the  primaries  or  of 
one  of  the  secondaries  of  the  transformers  should  be  reversed. 

The  lamps  which  are  used  with  the  synchronizer  should  be 
adapted  for  the  highest  voltage  which  they  will  receive.  Thusr  if 
they  are  placed  upon  the  secondaries  of  two  100-volt  transformers, 
there  should  be  two  100-volt  lamps  or  four  50- volt  lamps  in  series. 
If  two  200-volt  machines  have  the  lamps  applied  directly  without 
transformers  it  will  be  necessary  to  use  four  100-volt  lamps,  or  their 
equivalent. 

Rate  of  Pulsation  and  Size  of  Pulley.  The  difference  in  speed 
between  two  machines  may  be  determined  by  the  rate  of  pulsation 
of  the  synchronizing  lamps.  It  is  sometimes  convenient  to  know 
this  difference  in  speed,  especially  when  two  generators  are  belt- 
driven  from  the  same  shaft.  If  the  speeds  are  not  equal,  it  may  be 
necessary  to  turn  off  one  of  the  pulleys  in  order  to  make  them  equal. 
One  pulsation  of  the  lamps,  i.  e.,  the  interval  between  two  consecutive 
occurrences  of  maximum  brilliancy,  indicates  a  gain  of  one  cycle  or 
two  alternations  of  one  machine  over  the  other.  Thus,  if  there  is 
one  pulsation  of  brightness  per  minute,  and  the  number  of  alterations 
is  7,200  per  minute,  then  one  machine  gives  7,202  alternations, 
while  the  other  gives  7,200.  If  the  number  of  pulsations  of  bright- 
ness is  36  per  minute,  then  one  machine  gives  7,272  alternations, 
while  the  other  gives  7,200  alternations,  and  the  first  machine  is, 
therefore,  running  1  per  cent  faster  than  the  second  machine.  In 
order  to  determine  which  machine  is  running  the  faster,  the  load 
may  be  thrown  upon  one  machine,  or  its  belt  may  be  slackened  so 
as  to  decrease  its  speed.  If  this  be  done  to  the  machine  which  at- 
tempts to  run  too  fast,  the  pulsations  will  become  less  rapid;  while 
if  it  be  done  to  the  machine  which  is  running  slower,  the  pulsations 
will  become  more  rapid.  If  one  machine  is  running  1  per  cent  faster 
than  the  other,  it  will  be  necessary  to  reduce  the  diameter  of  the 
pulley  of  the  other  (slower)  machine  by  1  per  cent. 

The  thickness  of  the  belt,  the  tightness  of  the  belt,  the  slip- 
page (dependent  upon  the  kind  qf  belt,  the  condition  of  the  sur- 
faces of  the  pulley  and  belt,  and  the  load)  are  all  factors  which 
affect  the  speed  and  must  all  be  kept  in  mind. 


466    ,      ALTERNATING-CURRENT  MACHINERY 

Directions  for  Connecting  One  Alternator  in  Parallel  with 
Another  Alternator. 

(1)  Frequency.    The  speed  of  the  new  machine  which  is  to 
be  connected  in  parallel  must  be  made  such  as  to  give  the  same 
frequency  as  the  one  already  running.     If  the  latter  is  carrying  a 
load,  it  may  be  necessary  to  reduce  the  speed  of  the  unloaded  ma- 
chine below  that  at  which  it  tends  to  run  with  no  load,  by  adjusting 
the  engine  valve,  or  the  water  wheel  gate  or  nozzle,  or  the  belt  slip- 
page, in  order  to  secure  the  proper  speed. 

The  adjustment  of  the  engine  should  preferably  not  be  by 
throttling,  as  the  governor  is  liable  to  "hunt"  when  the  throttle  is 
opened.  It  is  desirable  to  be  able  to  adjust  the  governor  for  chang- 
ing the  speed  while  the  engine  is  running. 

(2)  Voltage.    The  field  excitation  of  the  new  machine  should 
be  adjusted  so  that  its  voltage  is  the  same  as  that  on  the  bus  bars, 
the  measurement  being  made  by  a  voltmeter. 

(3)  Phase    Coincidence.      Synchronizer    lamps    indicate    by 
their  slow  rate  of  pulsation  that  the  machines  are  of  practically 
equal  frequency.    When  the  synchronizer  lamps  indicate  the  proper 
phase  relation,  i.e.,  phase  coincidence  (preferably  when  the  lamps 
are  dark),  all  is  ready  for  closing  the  switch. 

(4)  Main  Switch.    Close  the  main  switch  a  little  too  soon  (when 
the  machines  are  approaching  the  proper  position)  rather  than  too 
late  (when  they  are  receding  from  it).     If  the  switch  is  operated 
by  compressed  air  or  for  any  other  reason  does  not  close  the  instant 
the  handle  is  operated,  due  allowance  must  be  made  for  the  interval. 

(5)  Equalizing  Switch.    If  the  generators  are  composite  wound, 
close  the  equalizer  switch. 

(6)  Adjustment  of  Load.    Adjustment  may  now  be  made  by 
means  of  the  governor  or  otherwise  so  that  each  machine  receives 
its  proper  share  of  load. 

(7)  Adjustment  of  Field  Currents.    The  field  currents  of  the 
several  generators  should  be  properly  adjusted  to  eliminate  cross 
currents  between  the  armatures  and  maintain  the  proper  voltage 
on  the  bus  bars. 

Directions  for  Cutting  Out  An  Alternator  Which  is  Running 
in  Parallel  with  One  or  More  Alternators. 

(1)   Preferably  cut  down  the  driving  power  until  it  is  just 


ALTERNATING-CURRENT  MACHINERY  467 

about  sufficient  to  run  the  generator  at  zero  load.    This  will  auto- 
matically reduce  the  load  on  the  generator. 

(2)  Adjust  the  resistance  in  the  field  winding  of  the  machine 
which  is  to  be  cut  out  until  its  armature  current  is  a  minimum. 

(3)  Open  the  main  switch,  then  the  equalizer  switch. 

It  is  usually  sufficient,  however,  to  simply  disconnect  a  machine 
from  the  bus  bars,  thereby  throwing  all  the  load  suddenly  on  the 
remaining  machines,  without  having  made  any  special  adjustments 
of  the  load  or  the  field  current.  The  objection  to  this  method  is  that 
it  may  cause  serious  hunting  of  the  remaining  machines. 

The  field  circuit  of  the  generator  to  be  disconnected  from  the 
bus  bars  must  not  be  opened  before  the  main  switch  has  been  opened; 
for  if  the  field  were  opened  first,  a  heavy  current  would  flow  between 
the  armatures. 


INDEX 


A  Page 

A.  c.  circuit,  fundamental  equation  of    32 
Air-brake  switches  436 

All-day  efficiency  265 

Alternating     currents,     advantages 

and   disadvantages  of      4 
a.  c.  machines,  miscellaneous  6 

a.  c.  machines,  simple,  construc- 
tion of  4 
high  e.m.f  s,  transformation  of  5 
Alternating-current  machinery        1,  467 
appendix                                           462 
alternating  into   direct  current, 

conversion  of  317 

alternating    electromotive    force 

and  currents  1 

alternators  97 

armature  windings  135 

commercial  types  of  machines       145 
induction  motor  371 

measurement  of  power  128 

measuring  instruments  60 

motor-generators  364 

rotary  converter  325 

switchboard   and  station  appli- 
ances 420 
synchronous  motors  216 
transformer                                        233 
Alternating  electromotive  forces  and 

currents  1 

a.  c.  circuit,  fundamental  equa- 
tions of  32 
alternating      currents,      advan- 
tages   and    disadvan- 
tages 4 
alternating   e.  m.  f's     and     cur- 
rents                                  12 
d.  c.  and  a.  c.  calculations               10 
d.  c.    and   a.  c.   problems,    com- 
parison of  7 
capacity                                              30 
condenser,  capacity  of                      31 


Page 
Alternating  electromotive  forces  and 

currents 
condenser    as    compensator    for 

lagging    current  47 

circuits  in  parallel  51 

circuits  in  series  50 

electrical  resonance  41 

electromotive  force,  average 
and  effective  values 
of  14 

electromotive     force     losses     in 

alternator  56 

electromotive     force     losses     in 

transmission  lines  56 

electromotive    force,    variations 

of  3 

graphical  representations  of  12 

harmonic     electromotive    forces     . 

and  currents  16 

inductance  27 

maximum  and  effective  values, 

relation  between  24 

power,    instantaneous  and  aver- 
age 15 
simple  alternator  1 
speed   and   frequency,    relations 

between  3 

Alternating  into  direct  current,  con- 
version of  317 
aluminum  valve  rectifier                 318 
mercury-vapor  arc  rectifier            320 
motor-generators  364 
rectifying  commutator                     317 
rotary  or  synchronous  converter  325 
Alternator  testing  188 
alternating-current     testing     in 

general  188 

armature  resistance  198 

characteristic  curves  193 

saturation  193 

synchronous  impedance  196 


INDEX 


Page 
Alternator  testing 

core  loss  and  friction  test  209 
efficiency,  calculation  of  214 
heat  test  205 
insulation  testing  188 
break-down  191 
dielectric  strength  189 
resistance  189 
regulation  200 
American  Institute  rules  203 
curve  202 
Alternators  97 
armature  inductance  106 
armature  reaction  104 
armature  windings  135 
commercial  types  145 
economy  factors  179 
electromotive  force  lost  in  arm- 
ature drop  107 
field  excitation  109 
fundamental  equation  of  97 
measurement  of  power  128 
parallel  operating  of  462 
polyphase  alternators  118 
single-phase  system  118 
three-phase  system  121 
two-phase  system  118 
regulation  of  108 
Aluminum  valve  rectifier  318 
American  Institute  rules  184 
limiting  temperature  rise  185 
overload  capacities  187 
rating  184 
Appendix  462 
Armature  drop,  e.  m.  f.  lost  in  107 
Armature  inductance  106 
Armature  reaction  104 
Armature  windings  135 
classification  135 
according  to  construction  of 

core  136 
according    to    disposition    of 

coils  139 
according    to    form    of    con- 
ductor 141 
according   to   progression   of 

winding  139 
according  to  shape  of  core       135 


Armature  windings 
single-phase 
three-phase 
two-phase 

Auto  transformer 


B 


Bar  winding 


Circuit  breakers 

Circuit  interrupting  devices 

air-break  switches 

circuit  breakers 

feeder  or  voltage  regulators 

fuses 

lightning  arresters 

oil-break  switches 

voltmeter  compensator 
Commercial  types  of  machines 


Page 

142 
143 
143 
251 

142 


438 
435 
436 
438 
444 
435 
452 
440 
450 
145 


revolving-armature  alternators  147 
revolving  field  alternators  154 
Compensated  wattmeter  79 
Condensers,  capacity  of  31 
Constant-current  transformer  293 
Cooling  of  transformer  292 
self-cooling    dry    transformers  293 
self-cooling,       oil-filled       trans- 
formers 294 
water-cooled  transformers  296 
Core-loss  and  exciting-current  test  310 
Core  type  transformer  271 
Current    relations    for    rotary    con- 
verter 336 
Curtis  turbo-alternator  175 
Customer's  bills,  calculations  of  95 

D 

Dielectric,  inductivity  of  31 
Direct-      and      alternating-current 
problems,    comparison 

of  8 

alternating  current  8 

direct  current  7 

Direct-current  voltage,  control  of  349 

E 

Economy  factors  in  alternators  179 

alternator  testing  188 


INDEX 


Page 
Economy  factors  in  alternators 

conditions  affecting  cost  179 

frequency  180 

regulation  180 

speed  179 

voltage  179 

efficiency  181 

power  losses  181 

rating  and  overload  capacities  183 

Efficiency  of  alternator  181 

influence  of  power  factor  upon 

output  183 
practical  and  ultimate  limits  of 

output  182 
Efficiency  calculation  314 
Electrical  resonance  41 
multiplication  of  current  by  46 
multiplication  of  e.  m.  f.  by  44 
Electrodynamometer  72 
used  as  an  ammeter  73 
used  as  a  voltmeter  74 
Electrolytic-cell  lightning  arrester  456 
Electromagnetic  ammeters  and  volt- 
meters 67 
Roller-Smith  repulsion  ammeter  70 
Thompson  inclined-coil  meter  70 
Electromotive    force,    average    and 

effective  values  of  14 
Electromotive     force     relations     for 

rotary  converter  336 
Electromotive  force,  variations  of 

cycle  3 

frequency  3 

period  3 

Electrostatic  ground  detector  67 

Electrostatic  voltmeter  64 


Feeder  panels  430 

Feeder  or  voltage  regulators  444 

induction  448 

magnetic  446 

ratings  of  449 

Stillwell  444 

Field  excitation  109 

Field  excitation  and  power  factor  226 

Field  excitation  of  rotary  converter  353 

Form  factor  15 


Fort  Wayne  single-phase  alternator  147 

Four-ring  rotary  converter  328 

Frequency  changes  397 

Fuse  blocks  transformer  305 

Fuses  435 


General  Electric  rotary  converter  343 
General  Electric  three-phase  alter- 
nator 152 

H 

Harmonic  electromotive  forces  and 

currents  16 

addition  of  21 

algebraic  representation  19 

clock  diagram  representation  17 

graphical  representation  18 

phase  difference  19 

subtraction  of  24 

synchronism  19 

Heat  run  361 

Heat  test  308 

Hot-wire  ammeter  and  voltmeter  60 

Hunting  action  223 

Hunting  of  rotary  converter  345 

I 

Impedance  311 

Indicating  instruments  60 

elect  rodynamometers  72 
electromagnetic    ammeters    and 

voltmeters  67 

electrostatic  voltmeter  64 

hot-wire  ammeter  and  voltmeter  60 

induction  instruments  75 

wattmeter  77 

Inductance  27 

formulas  for  29 

series  and  parallel  30 

Inductance  generator  397 

Induction  instruments  75 

Induction  motor  371 

action  of  375 

behavior  at  starting  and  in 

operation  385 

efficiency  and  rotor-resistance  379 

efficiency  and  speed  376 


INDEX 


Page 

Induction  motor 
action  of 

mechanical  to  electrical  en- 
ergy in  rotor,  ratio  of  377 
rotor  for  constant  and  vari- 
able speed  380 
rotor  voltages  to  stator  volt- 
ages, ratio  of  378 
rotor  windings,  starting  resis- 
tance in  376 
torque  and  speed  375 
typical  induction  motor,   struc- 
tural details  of  380 
constructive  elements  371 
frequency  changer  397 
induction  generator  397 
installations  389 
single  phase  393 
hand  starting  393 
repulsion  motor  starting  395 
split-phase  starting  394 
stator  windings  and  their  actions  372 
tests  406 
break-down  408 
core  loss  410 
efficiency  413 
heat  406 
impedance  412 
performance   curves  of  416 
slip  414 
starting  torque  409 
Induction  watt-hour  meter  85 
meter  friction,  compensation  for  91 
moving  element  89 
polyphase  93 
power  factor  adjustment  89 
single-phase  86 
Inductivity  of  dielectric  31 
Integrating  instruments  82 
customer's  bill,  calculation    of  95 
induction  watt-hour  meter  85 
sparks  gauge  95 
Thomson  watt-hour  meter  82 
watt-hour  meter  dials  93 
Inverted  rotaries  347 
Iron  losses  262 

J 

Joule's  law  7 


K 


Kirchoff  s  law 


Page 

7 


Lightning  arresters  452 
condenser  with  choke  coil,  com- 
bination of  459 
electrolytic-cell  456 
installing,  instructions  for  460 
ground  460 
inspection  461 
insulation  460 
location  460 
lightning,  effects  of  452 
multi-gap,  non-arcing  454 
multi-path  455 
Lincoln  synchronizer       .  433 

M 

Measuring  instruments  60 
indicating  60 
integrating  82 
recording  60 
Mercury-vapor  arc  rectifier  320 
Motor  generators  364 
comparisons  of  with  rotary  con- 
verter 365 
types  367 
uses  of  366 
Motor  testing  229 
break-down  test  231 
phase  characteristic  229 
pulsation  test  230 
self -starting  test  231 
Multi-gap,     non-arcing     lightning 

arrester  454 

Multi-path  lightning  arrester  455 

Multipolar  rotary  converter  346 


Ohm's  law  7 

Oil-break  switches  440 

Oscillators  for  rotary  converters  342 

Outdoor  transformers,  mounting  of  308 


Parallel   operating   of   alternators  462 
connecting  one  alternator  in  par- 
allel with  another  466 
equalizing  switch  466 


INDEX 


Page 

Parallel  operating  of  alternators 
connecting  one  alternator  in  par- 
allel with  another 

field  currents,  adjustment  of  466 

frequency  466 

load,  adjustment  of  466 

main  switch  466 

phase  coincidence  466 

voltage  466 

cutting  out  alternator  466 

necessary  conditions  462 

frequency  462 

phase  462 

voltage  462 
relative    frequency    and    ohase 

coincidence  462 
pulsation  and  size  of  pulley  465 
synchronizer  463 
Phase  difference  19 
Phase  transformer  257 
Polarity  test  315 
Portable  torsion  wattmeter  80 
Power,  expression  for  26 
Power,  measurement  of  128 
balanced  systems  129 
three- wire  three-phase  131 
three-wire  two-phase  129 
unbalanced  systems  131 
four- wire  two-phase  131 
six- wire  three-phase  132 
three-wire  three-phase  133 
three-wire  two-phase  132 
Power  factor,  influence  of  upon  out- 
put 183 
Power  law  7 
Power  losses  181 

R 

Rating  and  overload  capacities  183 

American  Institute  rules  184 

limiting  temperature  rise  185 

overload  capacities  187 

rating  184 

Rating  of  transformers  267 

Rectifying  commutator  317 

Regulation  312 

Regulation  of  alternator  108 

Resistance  of  coils  309 


Page 

Revolving-armature  alternators  147 

field  structure  for  Westinghouse 

180-kw.  152 

Fort  Wayne  single-phase  147 

General  Electric  three-phase          152 
Westinghouse  armature  with  dis- 
tributed winding  151 
Westinghouse  uni-coil  armature    150 
Revolving-field  alternators  154 
construction  151 
armature                                      155 
armature  coil                               157 
bed  plate  and  bearing  pedestal  1 56 
excitation                                     163 
field  coils  163 
frame  1 54 
rotating  field                                      159 
supporting  ring  158 
terminals  159 
steam-turbine-driver  169 
water-wheel-driver  Ifi5 
Roller-Smith  repulsion  ammeter  70 
Rotary  converters  in  practice              3118 
characteristic  types  of                     343 
direct-current  voltage,  control  of  349 
with  Edison  three-wire  system      355 
field  excitation                                   353 
hunting  of                                            345 
inverted                                              347 
oscillators  for                                     342 
six-phase                                               357 
starting  of                                          339 
transformer  connections  for  uses 

of  338 

Rotary  or  synchronous  converter       325 
comparison    with    direct-current 

dynamo  326 

current  relations  for  336 

direct-current    dynamo    into    a 

rotary  converter  327 

e.  m.  f.  relations  for  330 

four-ring  329 

multipolar  330 

in  practice  338 

six-ring  330 

testing  of  361 

heat  run  361 

standard  300 


INDEX 


Page 
Rotary  or  synchronous  converter 

three-ring  329 


Saturation  curve                                  '  193 
Scott  transformer  259 
Series  or  current  transformer  297 
Shell  type  transformer  283 
Simple  alternator  1 
Single-phase  alternator  118 
Single-phase  windings  142 
Six-phase  rotary  converter  357 
Six-ring  rotary  converter  330 
Spark  gauge  95 
Speed  and  frequency,  relations  be- 
tween 3 
Speed-limiting  devices  348 
Steam-turbine-driven  alternators        169 
advantages  170 
Curtis  turbo-alternator  175 
rotor  171 
stator  171 
Strap  winding  142 
Synchronism  19 
Synchronous  impedance  curve  196 
Synchronous   motor   and   induction 

motor,  comparison  of     401 

Synchronous  motors  216 

advantages  218 

compared  with  d.  c.  motors  219 

disadvantages  219 

field  excitation  and  power  factor  226 

hunting  action  223 

motor  testing  229 

starting  motor  220 

exciter  220 

polyphase  220 

polyphase  type,  self-starting 

of  221 

separate  starting  221 

single-phase  circuit  220 

torque  and  power  output  225 

use  as  a  condenser  227 

Switchboard  apparatus  433 

circuit  interrupting  devices  435 

lightning  arresters  452 

Lincoln  synchronizer  433 

Switchboards  420 


Switchboards 
feeder  panels 
high  voltage  panels 
polyphase 
typical  single-phase 


Page 

430 
433 
425 
422 


Tables 

capacities  of  standard  transform- 
ers 419 
constant-current         transformer 

data  300 
A  and  Y  connection  data  in  mains  127 
A  and  Y  connection  data  in  re- 
ceiving circuits  128 
inductivities  of  dielectrics  31 
power  ratings  of  rotary  conver- 
ters in  kilowatts  327 
size  and  cost  of  copper  wire — 

two-wire  system  5 
sparking    distances    for    various 

voltages  192 
transformer    efficiencies,    losses, 

etc.  263 
turbine  speeds  for  alternators  170 
voltage  ratios  of  rotary  conver- 
ters 335 
Thomson  inclined  coil  meter  70 
Thomson  watt-hour  meter  82 
Three-phase  alternator  321 
A-connected  armatures  125 
current  relations  125 
electromotive  force  relations  125 
receiving  circuits  126 
dissimilar      circuits      (unbal- 
anced system)  126 
similar  circuits  (balanced  sys- 
tem) 126 
Y-connected  armatures  124 
current  relations  125 
electromotive  force  relations  124 
Three-phase  transformers  289 
Three-phase  windings  143 
Three-ring  rotary  converter  329 
Torque  and  power  output  225 
Transformer  235 
automatic  action  of  238 


INDEX 


Page 
Transformer 

coil     resistance     and     magnetic 

leakage  242 

commercial  types  of  270 

constant  current  299 

fuse  blocks  305 

cooling  of  292 

core  271 

series  or  current  297 

shell  283 

three-phase  280 

connections  242 

auto-transformer  251 

auto      step-down     trans- 
formation 252 
auto  step-up  transforma- 
tion                                  252 
Current  relations                   253 
parallel  -  constant  -  voltage 

transformers  242 

banking  of  247 

Edison  three-wire  system 

single-phase  246 

multi-coil  type  243 

series-current  transformers       249 
description  233 

ideal   action    graphically    repre- 
sented 239 
ideal  and  practical  206 
maximum  core  flux                          237 
physical  action                                 234 
current  relations                         235 
electromotive  force  relations    234 
with  load                                     234 
without  load                               234 
polyphase  systems                           254 
with     compound     magnetic 

circuits  257 


Page 
Transformer 

polyphase  systems 

phase  transformer  257 

three-phase  255 

two-phase  255 

practical  consideration  262 

efficiency  263 

rating  of  267 

regulation  266 

transformer  losses  262 

regulation  241 
with  highly  inductive  load       241 

with  non-inductive  load  241 

tests  308 
core-loss  and  exciting-current  310 

efficiency  calculations  314 

heat  308 

impedance  311 

polarity  315 

regulation  313 

resistance  of  coils  310 
Transformer  connections  for  rotary 

converters  357 

Two-phase  alternator  118 

Two-phase  windings  143 


Voltmeter  compensator 


W 


450 


Water-wheel-driven  alternators  165 
Watt-hour  meter  dials  93 
Wattmeter  77 
Westinghouse  armature  with  dis- 
tributed winding  151 
Westinghouse  uni-coil  armature  150 
Wire  winding  141 


°" 


THE 


NOV 


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UNIVERSITY  OF  CALIFORNIA  LIBRARY 


